Three related sources let conduct and context inform the meaning of a contract, particularly under the Uniform Commercial Code.
Course of performance is how the parties have behaved under this contract — repeated occasions for performance, accepted without objection. It is the most persuasive of the three because it shows what these parties understood this agreement to require.
Course of dealing is their conduct under previous contracts with each other, establishing a common basis of understanding.
Trade usage is a practice so regularly observed in a place, vocation or trade that parties can be expected to have it in mind. A party need not know the usage to be bound if they operate in the trade.
Priority. Where they conflict, express terms control course of performance, which controls course of dealing, which controls trade usage. But courts construe them as consistent with each other wherever reasonably possible, and that construction is where a great deal of practical give resides.
The waiver risk. Accepting non-conforming performance repeatedly without objection is course of performance and can also be waiver. Contracts respond with anti-waiver clauses, which help but are not absolute — a sufficiently consistent course of performance has been held to waive the anti-waiver clause itself.
Discipline is the answer. Object in writing, promptly, every time, or accept that the practice has become the contract.