Witness share of block size
how much of the block's bytes were witnesses
What fraction of the block's raw bytes are signature and script data. It is a different question from the share of the weight budget, because witness bytes are discounted.
DefinitionWitness bytes / block size in bytes. This is the ratio written out in METRIC-IDEAS s3.14; it is a different number from the witness share of weight and both are published.
LimitIt is a linear mirror of the weight discount (r = -1.000), so those two are one measurement.