With every protocol you purchase, you will be able to see a complete list of fields and methods. Each protocol definition (the RFC or equivalent) is designed to include all available fields and methods, and you have the ability to review the protocol definition and make a request to add anything missing (extensions, etc.).
You can also open the module browser to view beSTORM decision tree which displays all relevant test combinations:
The full specs for protocols are not available prior to purchase. However, the total number of combinations is around 600 million for DHCP. See the attached PDF for the protocols covered in the DHCP module.