For jobs based on written employment contracts, what the agreement says typically governs all the terms and conditions of employment. If something is unclear or unstated, what the parties do later likely will influence eventual judicial interpretation.
Bottom line: If you spell it out in the agreement, you can avoid ambiguity and litigation.
Recent case: Ryan, who sold drilling equipment, worked under an agreement provided for $4,000 per month. plus commission. However, it didn’t spell out the commission structure. Ryan often received a 3% commission, but sometimes less. When he was fired, he sued, alleging he should have been paid 3% on all sales.
A jury heard testimony from both Ryan and the company. It concluded the commission rate wasn’t set by contract, but by past practice. Ryan lost. (Colter v. Amkin Technologies, No. 01-14-00175, Texas Court of Appeals, 2015)