Position limits are the ceilings a broker sets on how much exposure one account may carry: a maximum lot size per order, a maximum number of open and pending orders, and a total volume cap per instrument. They live in the contract specification, and on some servers they tighten automatically as account equity or total exposure grows.
They are almost always met at the worst moment — scaling into a position that is working, or trying to hedge one that is not. A rejection at that point is not a platform fault but a rule that went unread, and the message the terminal returns is often as unhelpful as "invalid volume". Limits also differ between account types at the same broker, so switching account can silently move the ceiling under a strategy that depends on size.
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Guide
How to Measure Risk Across All Your Open Positions
Risk per trade only describes one ticket. How to add up what your stops are worth, split the book into currencies, and read the margin figures the platform is already showing you.