Multi-select custom fields allow you to add more than one custom field to the object you want. In other words, you can assign multiple custom fields to a person, project, or invoice.

Multi-select custom fields can only be used as dropdowns.

To add a multi-select custom field, go to Settings > Custom Fields > select the object you want to add it and click on Add new custom field:

From the menu, select Multi-select field:

Be sure to define the name, add a description and add all the options (and add colors to the field):

Hit Create field when you're done.

Now, let's see how to use it in practice: Because the field from the example above can be assigned to people, it can be used in Contacts > People:

Now let's take a look at how the Filters look:

Notice that they've changed because now an object can have more than one value. For example, using this filter, I'll see people who have Graphic design as one of their skills:

However, if the filter is set to equals ( = ) it will show only the people who fit the exact criteria:

The same applies to multiple custom fields assigned: