Unknown said:

The "type" and "x" attributes are optional, so this is only specific to TMX files generated by SDL.

BTW, Jesse Good, the optionality of "x" attribute just means that "it's not mandatory to be present in each and every tag".
But it still means that - given its purpose to provide cross-reference between source and target tag - it SHOULD (or even MUST?) be present if necessary!

What that "necessary" means?
Basically, if there is only single tag in both source and target segment, it's not necessary to provide a clue to link them together, obviously...
But if there is more tags, then it is necessary to provide information which is which... because the word order may be different in source and target language, so e.g. first tag in source sentence may not necessarily correspond to first tag in target sentence. So you need the "x" attribute to provide this clue...

Unknown said:

The "type" and "x" attributes are optional, so this is only specific to TMX files generated by SDL.

BTW, Jesse Good, the optionality of "x" attribute just means that "it's not mandatory to be present in each and every tag".
But it still means that - given its purpose to provide cross-reference between source and target tag - it SHOULD (or even MUST?) be present if necessary!

What that "necessary" means?
Basically, if there is only single tag in both source and target segment, it's not necessary to provide a clue to link them together, obviously...
But if there is more tags, then it is necessary to provide information which is which... because the word order may be different in source and target language, so e.g. first tag in source sentence may not necessarily correspond to first tag in target sentence. So you need the "x" attribute to provide this clue...