Why might a Cessna 208 need a FAR Part 23 waiver?
The Cessna 208 is a bit of a confusing model when it comes to seating.

Short version:

Under US legislation, the aircraft may have up to 11 seats and can carry 9 passengers and 2 pilots. Other countries did recertify the aircraft to have up to 14 seats. By doing so they adopt the FAA certification based on part 23 and add text to waive the 9 passenger limitation.



Long version:

Originally the type was approved by the FAA in October 1984 under FAR 23 (up to amendment 28) and given FAA Type certificate number A37CE. The aircraft is certified as 11 Place Closed Land Monoplane (PCLM) and later as 11 Place Closed Sea Monoplane (PCSM).

In 1984 the applicability of Part 23 was described by 23.1 (since amendment 10) as:


  (a) This Part prescribes airworthiness standards for the issue of type
  certificates, and changes to those certificates, for small airplanes
  in the normal, utility, and acrobatic categories that have a passenger
  seating configuration, excluding pilot seats, of nine seats or less.


So this is where the 9 passenger limitation originally comes from. This also implies that if you operate with only one pilot, one of the 11 seats must be unoccupied.

Until 1971 there was no mention of a 9 passengers limitation and since 1987 (amendment 34) a new category of commuter planes is defined allowing for more than 9 passenger (up to 19 passengers) in propeller-driven, multiengine airplanes.

Since the Cessna 208 is a single engine aircraft, the commuter category cannot apply. As a result, when the aircraft is operating under the FAA type certificate, there is a limit of 9 passengers. 

When the aircraft is registered and operated outside the USA, the FAA rules do not necessarily apply. Usually a country adopts the Type Certificate of the original certifying country, but changes can be made. This is exactly what happened with the Cessna 208. 

Since the aircraft offers space for more seats and the maximum take-off weight allows for extra passengers, Cessna started to offer a 14 seat model for use outside the USA.  

For aircraft with more than 11 seats, the flight manual has a supplement stating:


  Limitations
  
  Utilization of the 13-place, 14-place and Utility seating options
  described in this supplement is limited only to nations where approval
  has been received. The United States does not authorize more than 11
  seats in the airplane. In the 11- seat configuration, the right front
  seat may be occupied by either a second crew member or a passenger,
  However, when the right front seat is occupied by a passenger, only
  eight seats in the aft cabin can be occupied.


Europe (EU)

The EASA Type certificate allows the 11 seat model 208. It also mentions a 14 seat model 208B being approved by the UK CAA. When EASA took over aircraft certification in Europe, the UK CAA approvement for 14 seats was adopted, but only for the 208B model. 

In section 2.E (only for 208B):


  A 14 seat place configuration was certified before 28 September 2003
  by CAA UK (see UK TCDS FA54) and subsequently adopt by EASA. Those
  aircraft have to be operated i.a.w. FAA approved AFM 1329-3- 13PHUK
  and AFM Supplement D1340-1-13UK or latest approved revision.


The original UK CAA approval included the text:


  MAXIMUM OCCUPANTS/PASSENGERS: 14 occupants, including pilot. Refer to
  British Aeroplane Flight Manual


Australia

The type certificate for the Cessna 208 mentions:


  Australian registered Cessna 208 aircraft may be configured for
  greater than 9 passenger seats. The requirement of FAR 23 paragraph
  23.1, introduced by amendment 23-10, to limit the passenger seating configuration, excluding pilot seats, to nine seats or less is not
  applicable to Australian aeroplanes certificated under Section 101.22
  of Civil Aviation Orders.


In the type certificate for the Cessna 208B reference is given to the UK CAA approval for more than 11 seats.

New Zealand

New Zealand allows the 208 to be operated with 14 seats. 


  In New Zealand the Model 208 Caravan has been approved for operation
  in the fourteen seat configuration in accordance with the provisions
  of the Fourteen Place Seating Supplement to the Model 208 Pilot’s
  Operating handbook. (A similar Supplement is included in the Flight
  Manual for the 208B.) The 208 aircraft was “accepted on the basis of
  compliance with FAR23 excluding the wording of FAR 23.1(a); ‘that have
  a passenger seating configuration, excluding pilot seats, of nine
  seats or less’.”


South Africa

The type certification document from New Zealand mentions that at the time of first approval (1985) Cessna stated: 


  Australia, UK, South Africa, Norway, Sweden, Kenya and Columbia had
  all accepted the Cessna 208 with more than nine passenger seats.


Interestingly in February 2014 the CAA of South Africa (SACAA) denied that the 208 was ever approved for operation with more than 11 seats in South Africa. Operators using the Cessna 208 with more than 11 seats found themselves operating the aircraft illegally suddenly. 

Quickly a Supplemental Type Certificate (STC) was approved in June 2014 which allows operation with 14 seats again.