Squawk7777,
In Canada they have a nice feature in which they give not only the amount of cloud, but how much this could if effect your inflight visability.
The AC4 means that the BKN ALTOCUMULUS at FL110 is really only equivilant to covering 4/8 (i.e. very thin), it helps you work out if its overcast by a thick layer or cloud, or multiple thin layers.
A better example ....
METAR CYQL 011800Z 18018KT 30SM FEW060 FEW100 BKN210 00/M08 A2935 RMK SC1AC1CI3 SLP994
RMK SC1AC1CI3 means CLOUD WITH OPACITY: STRATOCUMULUS 1/8 (FEW060) ALTOCUMULUS 1/8 (FEW100) CIRRUS 3/8 (BKN210), i.e. it is not overcast as the combined opacity is not 8/8 even though the combined cloud eighths is greater than 8.
The SLP994 is the QNH in MB, A2935 in inHg
Z