Witness share of block weight
how much of the weight budget went on witnesses
What fraction of the block's capacity was spent on signature and script data. Witness bytes cost one weight unit each, so this is literally the share of the budget they took.
DefinitionWitness bytes / block weight. NULL before SegWit at height 481,824.
LimitIt is a different number from the witness share of block SIZE, and both are published. Do not mix them up.