Under custom, an oath such as “I will not buy heads” need not encompass every animal or bird head. Established marketplace usage determines the relevant conventional class.
«دلالة العرف»
An indication arising from established custom.
«كل مملوك لي فهو حر»
Every person owned by me is free.
This illustrates an indication within the wording itself. The book treats the mukatab—an enslaved person under a contract of emancipation—distinctly because the combined wording evokes complete ownership. The scope is inferred from the full expression, not an isolated dictionary meaning.