The FIX Tag=Value encoding is the original encoding and the most widely used encoding. A simple ASCII string format.
The FIX Tag=Value encoding is specified in FIX 5.0 Service Pack 2 Volume 1 specification document – FIX Tag=Value Syntax. A separate document of the Tag=Value specifications is being developed.
FIX 5.0 SP2 Volume 1 Specification with 20110818 Errata 1.20 MB 825 downloads
Volume 1 of the FIX 5.0 Service Pack 2 (with 20110818 Errata) contains the FIX tag-value syntax rules and the FIXML Schema syntax rules in the "FIX Protocol Syntax" section - FIX Tag=Value Syntax and FIXML Syntax.Create date: July 24, 2017