This page describes the RFC mess with H263/+/++ RTP payloading and how Farsight deals with it.
We have 4 relevant RFCs :T
2190 - a "historic" RFC for payloading H263
2429 - an obsolete one for payloading H263+ specifically
4629 - a new one that obsoletes 2429. Used for payloading H263/+/++ (nearly identical to 2429)
4628 - makes 2190 historic
RFC2429 and RFC4629 are nearly identical. The difference is that RFC4629 says that plain H263 (1996) SHALL use it for payloading. It also defines some new SDP registrations for H263-1998 and H263-2000.
This means that all new implementations of plain H263 must use RFC4629 for payloading. It also says that when using RFC4629 for plain H263, we have to use H263-1998 without any optional annexes defined. If doing H263+, we have to use H263-1998 as encoding name and add the optional annexes we support. If we are using H263 payloaded with RFC2190, we have to use H263 as the encoding name.
The main problem with this is that UAs who don't understand RFC4629 will think that H263-1998 without annexes is actually H263+ as defined in RFC2429, but based on RFC4629 it is H263. So this means that UAs that implement RFC4629 could be incompatible with UAs that don't.
So we have the following encoding names :
H263 -> H263 with RFC2190
H263-1998 -> H263 with RFC4629 or H263+ with RFC2429
H263-1998 with annexes -> H263+ with RFC4629
H263-2000 -> H263++ with RFC2429 payloading or H263++ with RFC4629
Farsight deals with this by support RFC2190 for old implementations, and RFC4629. It will prioritize H263 over H263-1998 in order to maximize backwards compatibility with UAs that don't do RFC4629.