The {1:___} syntax is just Pro/E defining a 'block' of text, mostly for formatting purposes.
I've put these in manually on various occasions, for two reasons:
1) To isolate a section of text for a different format (e.g. larger text height). After putting the marker into the text and closing the dialogue box, you can first select the whole note but then select just the text within the marker; and then use Text Style to change it.
2) To clearly separate a parameter or dimension reference from adjacent text. For example, "CSK Ø&d21x&d22" doesn't work (IIRC - or didn't in some version of Wildfire) because it tries to look for a parameter called "d21x"; inserting the separators makes it clear: "CSK Ø{1:&d21}x&d22".
(In case you're not aware of this technique, it calls up the dimensions d21 and d22 within the current note (or other dimension) and displays as, for example: "CSK Ø15x90°".)
The number is just an identifier: typically the first 'block' is {1:___}, the next is {2:____} and so on.
Exactly why it's decided that your particular block of text needs isolating, I can't tell you... 
Also, why it appears to have changed the parameter reference to point to a different parameter is not obvious.
