For date/time functions that take a formatting picture string, the specification states:

   "The width modifier, if present, is introduced by a comma or semicolon. It takes the form:"

It then shows the form without a semicolon:

   ","  min-width ("-" max-width)?

And in the introduction of 9.8.4. (FO30 and FO31) it states:

   "2. The width modifier may be recognized by the presence of a comma."

Later in the text, it seems to only refer to a comma as an allowed width modifier introducer. I didn't readily find any tests that allow the semicolon in this position.

Looking back at the XSLT 2.0 specification, it only speaks of a comma.

It may be handy to allow a semicolon in this position (as the text suggests), in cases where you want the picture string to contain commas, you can use a single semicolon to introduce width, or vice versa, which seems more user-friendly than the rule that you must use a width modifier if you want to use a comma inside the token (i.e. as a thousands separator). 

Perhaps that was the original idea behind this, but the rest of the text does not reflect that.