Closed equity counts only completed trades. Open equity - sometimes called total or mark-to-market equity - includes what the live positions are currently worth. On a day with big open winners the two numbers can differ by 10% or more, and every percentage-based rule inherits that difference.
The consequence is compounding behaviour. Sizing off open equity grows positions faster in a winning run and cuts them faster when open profit evaporates, which increases both return and sequence-risk. Sizing off closed equity is steadier and slightly slower. Neither is wrong; using whichever is larger at the moment is.
Write the choice into the plan and apply it to every rule at once - sizing, max-open-risk, loss limits and the high-water-mark. Mixed bases are how traders accidentally run 30% hotter than they believe.
Related: sizing-on-closed-equity, sizing-on-open-equity, high-water-mark, sequence-risk