In the RICs for FX rates we see the following structure: for example for EUR GBP
What are the options that can be used after the "=" sign?
Where can I find documentation for that?
The easiest way is probably RIC Search Tool, and search for "EURGBP=".
However, as a developer, taking this a step further, you are probably not looking to find all the available rates, rather, the ones, that your organization is permissioned and intended to access? If I am guessing correctly, your Refinitiv account team would be the best contact, to find which rates your organization is permissioned for and should request.
The suffix after = defines the provider of the rate. Not every currency rate is provided by every venue, so for the definitive answer on the currently available, by specific venue, the best approach would be to contact Refinitiv content experts via Refinitiv Helpdesk Online -> Content