Bitcoin Private (BTCP) is a fork-merge of Bitcoin and ZClassic (ZCL, a fork of ZCash that removed the founders’ reward). BTCP defined its initial supply according to the sum of the outstanding supply of Bitcoin at the time (16.8m), ZClassic (3.4m) and a small 62,500-unit miner program. This was intended to give it an initial supply of ~20.4m BTCP, with a decaying miner reward, capping the total supply at 21m units as with Bitcoin.
The motivation for the creation of Realized Cap was the realization that “Market Capitalization” is often an empty metric when applied to cryptocurrencies. Market Capitalization, borrowed from the world of equities, is calculated for cryptocurrencies as
circulating supply * latest market price
However, unlike with equities, large fractions of cryptocurrencies tend to get lost, go unclaimed, or become otherwise inert through bugs.