$DynamicStringLocal:x:y:z

[Raeven]

As explained by Rhys

$DynamicStringLocal:0:1:2 [...snip...] the number specifies which local [variable] to get data from

the first is string index (0 based)
second one is STR# chunk ID
third one is Object ID​

There's a variant that is just $DynamicStringLocal:0 that just gets a string from dialog prim strings (STR#301) with index in local 0

ME: And whichever menu item is chosen [in the pie menu] is returned in parameter 0?

stack object id needs to be set to your desired value
before calling add pie menu string [Add/Change the Action String ]
see the 3 tile stage background object for an example on that