Does the TIS-B "hockey-puck" contain VFR mode-C targets with unconfirmed altitude?
The AIM covers most of the operations and limitations of TIS-B on page 4-5-18


  TIS−B is the broadcast of ATC derived traffic information to ADS−B
  equipped (1090ES or UAT) aircraft from ground radio stations. The
  source of this traffic information is derived from ground−based air
  traffic surveillance sensors. TIS−B service will be available
  throughout the NAS where there are both adequate surveillance coverage
  from ground sensors and adequate broadcast coverage from ADS−B ground
  radio stations. The quality level of traffic information provided by
  TIS−B is dependent upon the number and type of ground sensors
  available as TIS−B sources and the timeliness of the reported data.
  (See FIG 4−5−8 and FIG 4−5−9.)


According to the AIM all transponder equipped (A/C/S) aircraft are included 


  
  Only transponder−equipped targets (i.e., Mode A/C or Mode S transponders) are transmitted through the ATC ground system
  architecture. Current radar siting may result in limited radar
  surveillance coverage at lower 10/12/17 AIM Surveillance Systems
  4−5−19 altitudes near some airports, with subsequently limited TIS−B
  service volume coverage. If there is no radar coverage in a given
  area, then there will be no TIS−B coverage in that area.
  


section d. TIS-B Limitations offers some more info

The system is not perfect and unconfirmed altitudes may display as shadows


  (b) The ADS−B−to−radar association process within the ground system
  may at times have difficulty correlating an ADS−B report with
  corresponding radar returns from the same aircraft. When this happens
  the pilot may see duplicate traffic symbols (i.e., “TIS−B shadows”) on
  the cockpit display.


More on transponder requirements 


  (d) The TIS−B system only uplinks data pertaining to
  transponder−equipped aircraft. Aircraft without a transponder will not
  be displayed as TIS−B traffic.




So pilot A in his Cherokee with a mode S transponder will show up but pilot B in his cub with no electronics and only a hand held who may be on radar will not show up. 

This technical order covers the topic important phrase 


  TIS-B is the ADS-B In function that provides for the in-cockpit
  display of other aircraft (without ADS-B) in the vicinity that are
  being seen via FAA secondary radar. General observations can be made
  during the flight to verify that known radar targets are appearing in
  the correct relative position on the ADS-B display(s) in the flight
  inspection aircraft. Of course, if there are no such
  targets-of-opportunity in the area during the flight, the opportunity
  to check TIS-B in this way will not be possible.


So if you are coming up on secondary radar you will be included in the ADS-B/TIS-B broadcast. ATC receives 1200 codes with altitude (encoder provided) so they will show up, and in my recent experience flying in the northeast most VFR traffic does show up. Aircraft with no transponder will not show up (radar target only). Im not sure what happens in the case of no altitude reporting.