Has the same relation to L as UP does to P.
The problem of reachability in directed planar graphs lies in UL [SES05].
If UL = NL, then FNL is contained in #L [AJ93].
Has the same relation to NL as FNP does to NP.
Defined by [AJ93], who also showed that if NL = UL, then FNL is contained in #L.
Has the same relation to PL as SPP does to PP.
Contains the maximum matching and perfect matching problems under a pseudorandom assumption [ARZ99].
Contains UL.
Equals the set of problems low for GapL.
Has the same relation to UL as P/poly does to P.
Equals NL/poly [RA00]. (A corollary is that UL/poly is closed under complement).
Note that in UL/poly, the witness must be unique even for bad advice. UL/mpoly (as in BQP/mpoly) is a more natural definition, but this is a moot distinction here because [RA00] show that they both equal NL/poly.