Antenna models are used to reduce the positional noise due to the fact that Trimble GNSS antennas while very good, have biases that are in the mm range.
Where the antenna model is applied varies by the application, RTK or Post Processed
For Post Processed data the antenna type is stored in the DAT/T0x file, the Antenna model is NOT applied to the observations in the file. This allows the user to change the antenna type after the fact and use different antenna models when post processing.
Trimble was one of the first, if not the first, vendor to understand the importance of antenna models for RTK operation, due to the increased precision that it provides.
For RTK traditionally the rover applied the antenna model for the base and the rover. This has a number of issues when the base is using an antenna that the rover does not know about. The issues depend on the "correction" format in use, there will always be increased noise and possibly height biases when the rover does not have the antenna model for the rover. In the RTCM V3 case the rover will not start RTK operation when the base station antenna is unknown.
This is clearly a poor user experience. Therefore Trimble has since 2008 (V3.80+ firmware) solved this problem by having the base station apply the antenna model for antenna's made after this date at the base station to avoid all these issues in the field, when using CMR and CMRx format
With RTCM V3 the default behaviour is to operate in the same way as CMR/CMRx, but on some models of GNSS Receivers, like the NetRx's this behaviour can be changed to make it that the rover have to apply the base antenna model as well. This is mainly used in applications where RTCM is being used as a raw measurement format, and is not recommended for RTK operation.
In RTK, the rover antenna model is always applied at the rover.
The exact antenna model that is used is based on the models in use at the release of the GNSS firmware.