Use labels in a single select or multi select list field to separate and organize long lists. 


After adding options click + Add labels. 



Enter the name(s) for your labels and click OK.



Arrange the labels where you want them to appear within the list options. 



On form start and edit single select list field labels will display as shown in the screenshot below.



On form start and edit multi select list field labels will display as shown in the screenshot below.