Go to [Settings＞Additional fields] to set up. (If there is a need, users can change checkbox into drop-down menu. The data will not be affected by the existing information.

Take the customer maintenance page for example. The multi-select drop-down menu will only be expanded after clicking, and will be hidden after selection, and you can enter keywords to filter out matching options.

Take the additional fields of customer "more criteria" search for example. If it is a (multiple selection) drop-down menu, (multiple selection) selection box, option button, then multiple selections are allowed when querying. When there are multiple values ​​in a single additional field of the query, the judgment method is "or"; while between different additional fields is "and". 

Take the following picture for example. With this settings, you will find customers with purchase location in one of the choices, between "main" or "branch", and with "Bruce" as the sales representative.