export-filter/renditions argument - what is the 'plugin' field for?

For example:

local renditionsOptions = {

    pluginId = ...

    -- ...

}

for sourceRendition, renditionToSatisfy in filterContext:renditions( renditionOptions ) do

...

Doc says it'll be provided automatically if left nil, but can one provide another plugin Id or anything?

Or is it just some kinda "do nothing" leftover...?

Reserved for future maybe?

Any ideas??

R