This read-only Unsigned32, in MB, indicates DRE disk space currently used by the sigblock (signature block), which stores the metadata (chunk signatures) separately from the data units in the data block. An admin monitors this to see how much cache capacity is consumed by indexing overhead rather than actual cached payload data. If sigblock usage grows disproportionately compared to data unit usage, an admin might investigate whether traffic is producing unusually small, signature-heavy chunks.