Unavailable and missing ids to authority files

[morethanbooks]

I am not sure whether this is an issue or not, but in this case: https://raw.githubusercontent.com/COST-ELTeC/ELTeC-fra/master/level1/FRA01801_Gay.xml
<title ref="viaf:unavailable wikidata:missing">Ellenore : édition ELTeC</title>
I wonder whether unavailable and missing have really distinct meanings or if they could be replaced by a single solution, probably by missing. There are 11 cases of texts identified with unavailable and 66 with missing. Besides, there are 6 authors with missing and 0 with unavailable.
Regards!