languagesystem DFLT dflt

[bobh0303]

The rationale for the ufo_features_default_languagesystem check includes this suggestion:

there are cases where not having a languagesystem DFLT dflt statement in your feature file is technically correct.

Could someone help me understand what those cases are?

I ask because I build Arabic fonts that include a basic Latin set, so I already have:

languagesystem arab dflt ;
# and a bunch of other arab script language systems
languagesystem latn dflt ;

and I'm trying to figure out what possible reason there is for including languagesystem DFLT dflt.

As you know, the Adobe spec says:

This script/language pair is special: it is used if a program cannot find a match in the font to the current writing script and language. If it is not in your font, then all the rules may be invisible to the program if your font does not have a match for the current script and language.

As far as I can figure, "if a program cannot find a match in the font to the current writing script and language" then none of the rules in the font matter anyway so all rules probably should be invisible.