Params is used to pass the parameters of the template called to another template. It returns a bar | separated list of the parameters. So far it will only return up to any 20.

If {{Params|{{{1}}}}|{{{2}}}}|{{{3}}}}|{{{4}}}}} is called from inside a template call

{{foo|Param_1|Param_2|Param_3|Param_4|Param_5|Param_6}},
then it will return

Param_1|Param_2|Param_3|Param_4.

Concatenating calls to Params can be used return more parameters.

{{Params|{{{2}}}}|{{{3}}}}|{{{4}}}}}|{{Params|{{{6}}}}|{{{5}}}}}

will return

Param_2|Param_3|Param_4|Param_6|Param_5