Official Docs
The native multi-select field displays differently in lightning runtime (a single listbox) and classic runtime (dual side by side list boxes). It does not support ordering. It returns the selections as a string containing the choices separated by semicolons, which generally requires coding to parse.
There is a new unofficial Dual Listbox Flow Screen Component that provide ordering and a way to return choices as a collection of strings (which can then be looped over).
Blog Posts
http://www.sfdcr.com/automation/go-salesforce-flow-using-multi-select-picklists/
https://automationchampion.com/tag/parsing-multi-select-picklist-in-flow/
https://metillium.com/2018/09/flow-multi-select-picklist-selection-workaround/
