Declaration equations are usually given as part of declarations of constants or parameters. Such equations are actually required for constants, but are optional for parameters. An equation always holds, which means that the mass never changes value during simulation. It is also possible to specify a declaration equation for a normal state variable, but this will make the variable bahave as a kind of constant.
Modifier equations occur for example in a variable declaration when there is a need to modify the default value of an attribute of the variable. A modifier equation replaces the declaration equation, if present, for an attribute of the specific type. A common usage is modifier equations for the start attribute of variables, the equations also occur in type definitions.
Here is another example of a modifier equation: