if

The "if" is a function, with three parameters :

if ( condition : Boolean, trueResult : Any, falseResult : Any )

The condition is evaluated, and if it is true, then trueResult is returned, otherwise falseResult is returned.

The types for trueResult and falseResult should be the same.

e.g.

if ( CustomProperty.foo < Size.X / 2, 0mm, Size.X )

Can be read as : If foo is less than Size.X / 2, then return 0mm, otherwise return Size.X

And if that isn't clear enough, here's some equivalent pseudo code :

if ( foo < Size.X / 2 ) {
    return 0mm
} else {
    return Size.X
}