An order comment is a short piece of free text attached to an order when it is sent, most often used to record which rule or program placed it. It travels to the server with the order and generally appears against the resulting position and in account statements, which is what makes it useful after the fact.
It is a label rather than an identifier, and it is not fully under the sender's control. The field is short and longer text is truncated; brokers and servers may append to it or replace it, and a position closed by a stop or by an automatic process often carries a comment written by the server instead of the one it was opened with. Anything that has to be read back reliably by software therefore belongs in a field intended for that, with the comment left as what it is — a note for a human reading a statement later.
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Order Types and Slippage Explained
Market, limit and stop orders trade price certainty against execution certainty — plus what slippage is, where it comes from, and what a stop-loss caps.