I am working on a project where Jira will replace a custom solution and within that solution, fields are populated based on selections of other fields. What occurs is that when one field (Drop down single select) is chosen, another field is populated with results, however; those result can be modified within the request.
I've added a screenshot below to provide a better description (information has been altered to protect the innocent). Basically, what happens is a user will choose an affiliation. That selection will use the fields in the Available Access Points field to populated the Select Access Points field. However, if the user chooses, they can select specific options in either field to add or remove the access points.
So, I have one field that determines what the initial selection of access points will be populated in the Selected Access Points field, but we provide the option to customize the choices before the user officially submits the request.

If anyone has a suggestion or knows of an add-on that may serve my needs, please let me know. I cannot seem to find anything to support this need.