Closed equity is cash plus realised profit and loss. Sizing off it means a winning open position does not increase the size of your next trade until it is booked.
This is the conservative choice and usually the correct one for discretionary traders. Open profit is not yours yet: it can evaporate in a gap, and sizing off it means one reversal simultaneously shrinks your equity and reveals that everything you added was oversized. Compounding on paper gains is how good months turn into bad quarters.
Example: $50,000 closed equity with $8,000 of open profit. Closed-equity sizing risks 1% of $50,000 = $500. Open-equity sizing risks $580 and rises further as the winner runs. The difference looks trivial per trade and is not trivial across a correlated book.
Related: sizing-on-open-equity, compounding-position-size, open-trade-risk