Hi there!
Yes, you need to use that to change the HP/MP/TP parameters. But if you choose to use the short name instead of the parameter id, you need to use quotes, like this:
- $gameParty.members()[1].changeCustomHpParam("pwr", -2)
The difference between the two script calls you ask for is that using the actor one, you will target the actor by its ID property, as set on the database.
But using the member one, you will target the actor by its index position on your party. So:
Index 0 = First member on the party.
Index 1 = Second member of the party. and so on.