Daniel,
P and Q indicate the *type* of the entity
("P" = "Property", "Q" = "Item"
for
arcane reasons), "L" = Lexeme, "F" = Form, "S" = Sense,
"M" = MediaInfo).
As you
can tell, we'd quickly run out of letters and cause confusion if this
became
configurable.
I don't think this would cause a confusion, because the lexicographical
project is really a separate project that just happens to reside on the
same Wikidata domain. Essentially you did internally what we are asking for
other sites - you mixed two projects, and kept them distinct by using
different prefixes. If at some point you decide to add some new area of
data, e.g. biological, you could add new prefixes for that too, but that
would also be a "separate" project.
Most other sites that link to Wikidata only care about just one of those
projects. E.g. OSM would have very little interest in lexical data, so it
is OK if "L" prefix would be used in OSM and in WD because it won't be as
confusing to the users as reusing the Q.
The prefix can be omitted for local entities, so Q12345
is an item on the local repo (or the default repo of a wikibase client).
I think that was a big mistake -- the "(or the default repo of a wikibase
client)" -- because wd implies Wikidata, not Wikibase, so it dilutes the
meaning of "wd:". See my other email on how I fixed it.