I've seen this in a KTBs and U-boat diaries and assumed it meant the submerged depth of the sub in metres, with the 'A' being short for 'aufgetaucht' - surfaced. i.e. A-50 = 50m.
If you've seen A+20 in a KTB, then I may be wrong, unless the rumours were true about the secret flying U-boat, which could have won the war for the Germans.