mulscale <Result> <Factor 1> <Factor 2> <Divisor>
In C terms, this command performs roughly the expression
<Result> = (<Factor 1> * <Factor 2>) / <Divisor>.
In CON terms, it is very similar to the following series of commands:
set <Result> <Factor 1> mul <Result> <Factor 2> div <Result> <Divisor>
The main difference is that the calculation uses 64 bits to prevent overflow, while using gamevars for intermediate values would limit it to 31 bits.
For example, statusbarscale use a scaling factor of 100. If you wanted to scale the screen coordinates of a sprite, you would do:
scalevar Result Coordinate userdef.statusbarscale 100
Note that the equation above is very likely to surpass 31 bits. Using 16 bits of precision for rotatesprite, the maximum screen coordinate would be 327. So scalevar is required.