Hi @LN, as you can imagine, my silence does not reflect our interest for this great promising feature 
.
Thanks to both HOT and ODK teams for the work and thanks @Aly_Blenkin to make it visible 
before it existed 
I could copy/paste @seewhy 's response as it suits me fine !
About the vocabulary, "offline layer" is generic enough to be reusable in future features, with raster or vector layers (tiles or not).
I imagine to use it in combination with our nextcloud, to assist the user to transfer the files from the computer to the phone.