Hey Openczun,
What Derwyddon said, but to expand on it a little more. The letters indicate lifts that are grouped together with no rest between them. What isn't clear in my journal, the way I enter them, is the order and rest.
In the past I would do A1, A2, rest, A1, A2, rest, A1, A2, rest, B1, B2, rest, etc. Rest could be 1 -2 mins depending on the routine.
Currently, on heavy weight, low rep days it is A1, A2, rest, B1, B2, rest, A1, A2, rest, B1, B2, rest, etc. with rest=2 mins.
Circuit training days have no rest except for 2 mins between A & B, B & C. So they go A1, A2, A3, A4, A5, A1, A2, A3, A4, A5, A1, A2, A3, A4, A5, rest, B1, B2, B3, B4, B5, B1, etc. with rest=2 mins.