Hi,
the ISO15693 protocol is based on an exchange of
- a request sent by the reader
- a response from the tag
The data field in the SendRecv follows the request structure.
Each request consists of the following fields;
- Flags
- Command code
- Mandatory and optional parameters depending on the command
- Application data fields
- CRC
The ISO/IEC 15693-3 standard describes in details this protocol and the various mandatory and optional command codes. For custom and proprietary command codes, and for the list of optional command code being supported for a given tag, you will have to refer on the tag manufacturer data sheet.
For example, for ST25TV02K tags, see this Datasheet for details about supported command.
Rgds
BT