The answer can be found here:
The economic stake signalling a preference in the user PVRs refers to the percentage of the Bitcoin Days Consumed (BDC) in the last 3,000 blocks that were spent by transactions containing an MBSS in the nSequence field signalling a preference for a particular max block size value.
Bitcoin Days Consumed is an alias for Bitcoin Days Destroyed. Each transaction has a BDC that's calculated from multiplying the age of the UTXO it's spending by the value size of the UTXO.
So if you are spending a 1 year old 10 BTC transaction, the BDC would be 3650 (365 days * 10 BTC).