What parts of a circuit does an AFCI receptacle protect?
Arc-Fault Circuit Interrupter (AFCI) receptacles only provides sufficient arc fault detection and protection in the portion of the circuit downstream from the receptacle.

If AFCI receptacles offered adequate protection for the full circuit, you could install one receptacle and be done. However, NEC requirements for AFCI protection where an AFCI receptacle is used always require some additional form arc-fault protection for the portion of the circuit leading up to the first AFCI receptacle of the circuit.

The means of protection for the portion of the circuit leading up to the first AFCI receptacle include:


Combination-type AFCI breakers (here, no AFCI receptacle is required because the breaker provides sufficient protection for the entire circuit downstream from the panel). Refer to section 210.12 (A) 1 of the 2014 NEC.
Different types of supplemental arc protection breakers or AFCI breakers to detect arc faults in the portion of the circuit leading up to the first receptacle. Refer to sections 210.12 (A) 2-4 of the 2014 NEC. 
Nonflexible metallic conduit, MC, or some AC to protect against a fire caused by arcing with flammable materials in the portion of the circuit leading up to the first AFCI receptacle. Refer to section 210.12 (A) 5 of the 2014 NEC.
Metallic, nonmetallic conduit or tubing or MC cable encased in not less than 2 inches of concrete to protect against a fire caused by arcing with flammable materials in the portion of the circuit leading up to the first AFCI receptacle. Refer to section 210.12 (A) 6 of the 2014 NEC.
For individual branch circuits to a fire alarm system installed according to specific requirements in other parts of the code, including being fully enclosed in RMC, IMC, EMT, or steel sheathed cable, AC or MC with metal outlet and junction boxes, AFCI protection is not required in receptacle or outlet form because the entire circuit is protected against arc faults with flammable materials by the metal enclosure. Refer to the listed exception for section 210.12 (A) of the 2014 NEC.



  From the 2014 NEC:
  
  210.12 Arc-Fault Circuit-Interrupter Protection. Arcfault
  circuit-interrupter protection shall be provided as required
  in 210.12(A) (B), and (C). The arc-fault circuit interrupter
  shall be installed in a readily accessible location.
  
  (A) Dwelling Units. All 120-volt, single-phase, 15- and
  20-ampere branch circuits supplying outlets or devices installed
  in dwelling unit kitchens, family rooms, dining
  rooms, living rooms, parlors, libraries, dens, bedrooms,
  sunrooms, recreation rooms, closets, hallways, laundry areas,
  or similar rooms or areas shall be protected by any of
  the means described in 210.12(A)(1) through (6):
  
  (1) A listed combination-type arc-fault circuit interrupter,
  installed to provide protection of the entire branch circuit
  
  (2) A listed branch/feeder-type AFCI installed at the origin
  of the branch-circuit in combination with a listed outlet
  branch-circuit type arc-fault circuit interrupter installed
  at the first outlet box on the branch circuit. The first
  outlet box in the branch circuit shall be marked to indicate
  that it is the first outlet of the circuit.
  
  (3) A listed supplemental arc protection circuit breaker installed
  at the origin of the branch circuit in combination
  with a listed outlet branch-circuit type arc-fault circuit
  interrupter installed at the first outlet box on the branch
  circuit where all of the following conditions are met:
  
  
  a. The branch-circuit wiring shall be continuous from
  the branch-circuit overcurrent device to the outlet
  branch-circuit arc-fault circuit interrupter.
  b. The maximum length of the branch-circuit wiring
  from the branch-circuit overcurrent device to the
  first outlet shall not exceed 15.2 m (50 ft) for a
  14 AWG conductor or 21.3 m (70 ft) for a 12 AWG
  conductor.
  c. The first outlet box in the branch circuit shall be
  marked to indicate that it is the first outlet of the
  circuit.
  
  
  (4) A listed outlet branch-circuit type arc-fault circuit interrupter
  installed at the first outlet on the branch circuit
  in combination with a listed branch-circuit overcurrent
  protective device where all of the following
  conditions are met:
  
  
  a. The branch-circuit wiring shall be continuous from
  the branch-circuit overcurrent device to the outlet
  branch-circuit arc-fault circuit interrupter.
  b. The maximum length of the branch-circuit wiring
  from the branch-circuit overcurrent device to the
  first outlet shall not exceed 15.2 m (50 ft) for a
  14 AWG conductor or 21.3 m (70 ft) for a 12 AWG
  conductor.
  c. The first outlet box in the branch circuit shall be
  marked to indicate that it is the first outlet of the
  circuit.
  d. The combination of the branch-circuit overcurrent
  device and outlet branch-circuit AFCI shall be identified
  as meeting the requirements for a system
  combination–type AFCI and shall be listed as such.
  
  
  (5) If RMC, IMC, EMT, Type MC, or steel-armored Type
  AC cables meeting the requirements of 250.118, metal
  wireways, metal auxiliary gutters, and metal outlet and
  junction boxes are installed for the portion of the
  branch circuit between the branch-circuit overcurrent
  device and the first outlet, it shall be permitted to install
  a listed outlet branch-circuit type AFCI at the first outlet
  to provide protection for the remaining portion of
  the branch circuit.
  
  (6) Where a listed metal or nonmetallic conduit or tubing
  or Type MC cable is encased in not less than 50 mm
  (2 in.) of concrete for the portion of the branch circuit
  between the branch-circuit overcurrent device and the
  first outlet, it shall be permitted to install a listed outlet
  branch-circuit type AFCI at the first outlet to provide
  protection for the remaining portion of the branch
  circuit.
  
  Exception: Where an individual branch circuit to a fire
  alarm system installed in accordance with 760.41(B) or
  760.121(B) is installed in RMC, IMC, EMT, or steelsheathed
  cable, Type AC or Type MC, meeting the requirements
  of 250.118, with metal outlet and junction boxes,
  AFCI protection shall be permitted to be omitted.