Note the syntax of the if condition: Either side can use parameter placeholder.
If the parameter has a value like foo||bar i.e. created as a result of a request such as ?param=foo¶m=bar, then the equals=".." will check if any of the values match, and notequals=".." will check that none of the values match the value.
For the gt, ge, lt, le operators the comparison values will be treated as numbers (decimal). If either side are empty or not parseable as a number, the comparison is false.
The right hand side of isempty and hasmultiple can be true or false.