(For all my plugins, any parameter in the "Advanced" section should be ignored unless you know what you're doing with it. E.g. the "Save Property Name" parameter is for configuring how the plugin names its data in save files. It's there as a measure to mitigate conflicts with other plugins.)
As usual, Select Item shows a list of items in the party's inventory that match the specified category. The player can select one of those. Selecting an item from there does not use it: the command simply stores the ID of the selected item (or 0 if cancelled) in the variable you specified. You can then check the value of that variable and react accordingly.
In case it helps, here's an excerpt from a test event I have in my scrap project:
event excerpt
◆Change Items:Encounter Decreaser + 2
◆Change Items:Drop Increaser + 2
◆Change Items:HP Increase + 1
◆Text:None, Monster(1), Window, Bottom
: :Filtering for "potion"
◆Plugin Command:Cae_SelectItemFilter, Select Filter
: :Values = ["potion"]
: :Action = Set
◆Select Item:input, Regular Item
◆Text:None, Monster(1), Window, Bottom
: :Adding "epic"
◆Plugin Command:Cae_SelectItemFilter, Select Filter
: :Values = ["epic"]
: :Action = Add
◆Select Item:input, Regular Item
◆Text:None, Monster(1), Window, Bottom
: :Removing "potion" & "epic"
◆Plugin Command:Cae_SelectItemFilter, Select Filter
: :Values = ["potion","epic"]
: :Action = Remove
◆Select Item:input, Regular Item
◆Text:None, Monster(1), Window, Bottom
: :Adding "special"
◆Plugin Command:Cae_SelectItemFilter, Select Filter
: :Values = ["special"]
: :Action = Add
◆Select Item:input, Regular Item