It does work. In the control definition the Variable holding control's value is an array, not a string.
The user chooses multiple entries by ctrl/cmnd clicking on entries in the list.
You can see one use of such a list in the Open Selected Parts plugin in category Other.
Note this is available only in Sib 7.1.2 and later.
From: Plugin-dev [mailto:[hidden email]] On Behalf Of Thomas Weber
Sent: Thursday, July 20, 2017 8:14 AM
To: Sibelius plug-in developers <[hidden email]>
Subject: [Plugin-dev] Multi selection in dropdown lists
for dropdown lists, there is a "Allow multiple selections" option available. However, I have no idea how a user would be able to actually select multiple items. Does this actually work?
Notengrafik Berlin GmbH
UstID: DE 289234097
Thomas Weber und Werner J. Wolff