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The TS function parameter type that is determined by a different parameter is called a conditional type. It allows you to specify a type for a parameter based on another parameter or condition. This is done using the conditional operator (? :) within the type definition. For example:
type MyFunction<T, U> = T extends string ? (arg: U) => string : (arg: U) => number;
In this example, the MyFunction type takes two parameters T and U. If T is string, the function returns a string; if not, it returns a number. The type of the function's parameter arg is determined by the type of U. This is useful for creating generic types that have different behavior based on their input types.