Minor documentation issue:
There is slightly incorrect information listed for the documentation entry entitled About match fields for relationships.
1. The documentation says that "a match field can be a global field". This is actually not 100% true. A global field can ONLY be used as a match field if it is in the "source table", not the "destination table". (I'm making up the words "source" and "destination", but you know what I mean.)
2. The documentation also says that "Match fields must be able to be indexed." Again, this is not 100% true. It is perfectly fine for match fields to be UNINDEXED as long as they are in the "source table". The only time that they need to be indexed is in the "destination table".