I have not yet seen this double-hyphen explained, which is written eg. in An Etymological Dictionary Of The Hittite Inherited Lexicon (Kloekhorsts 2007)
-a (3rd.sg.pres.midd.-ending): see -a(ri)
=a 'and, too': see *=(i̯)a
=a 'but': see =(m)a
=a- (enclitic pronoun) 'he, she, it': […; pg. 196-198]
What's the difference from = to the usual hyphenation marking morpheme boundaries?
[…] The forms [n]=e-e=n (KBo 17.1 KBo 25.3 i 20 (OS)) and [n=e-e=]n (StBoT 25.4 i 15 (OS)) seem to show that […; pg. 208-209]
Wow, that's most cryptic. The question should be however easy to answer without detours into grammar (otherwise, pending own research, I would revise the question). Is the difference just ending versus enclitic?
How specialized is this, certainly not exclusive to Hittite studies?!