Electronic – What does “P. C. MOUNTING” stand for

BSC means "Basic Spacing Between Centers". This is often used if the reference lines don't refer to a physical point or edge, like in the case of the pins: the reference is in the middle of the pin, instead of one of the edges.

BSC SQ = BSC square, i.e. in both X and Y dimensions.

The abbreviation BSC is misused in the case of the package outline, and overall dimensions, as they are specified by physical edges which can be measured.

Usually a mechanical drawing of a package will show minimum and maximum values. BSC SQ must be regarded as nominal, so may deviate, but it doesn't say by how much.

The second is nominal, not mean. But yes, height is 0.95 mm minimum, 1.00 mm nominal, and 1.05 mm maximum.

The difference between mean and nominal is that nominal is the target dimension, that's the aim. Mean usually means arithmetic mean, but that may deviate from nominal, depending on spread.

This could be a statistic on the package thickness. Apparently the injection mold wasn't made precise enough, so that the mean thickness is 1.01 mm (purple line). But the nominal thickness is still 1.00, with 0.95 mm and 1.05 mm as limits. So even with a mean deviating from nominal that condition is met.