When you want to enter a name to an operator or function almost the same rules apply as for variable names. The only difference is the default scope. For variable names, the default scope is always the physics scope, represented by the phys prefix. When you declare a new operator or function, the default prefix is also phys, but not when you use the operator or function in an expression. Then the default is the comp prefix, which is interpreted as component scope. The reason is simply that it is most common that you declare new operators with physics scope, but not when you use a function. Then you often refer to functions that are unscoped (for example, sin, cos, exp, gradient, and normalize). In the Model Builder, all unscoped names are first interpreted using component scope, then root scope, so it is possible to change the meaning of the function name sin if you want.