If your library uses prefixes or suffixes in call numbers, you can build a standard list which will be displayed as drop down options when creating or editing call number records in the holdings editor.
This is how a prefix or suffix list appears when creating or editing a call number.

Click New Call Number/Volume Prefix or New Call Number/Volume Suffix.

Click Save.

Click Edit Selected.

Edit the value for Label, then click Save.

Changing a call number prefix or suffix immediately updates all call numbers using the prefix or suffix. While this may be desired, it can also cause a mismatch with physical item labels.
Click Delete Selected.

When the pop-up appears, click Confirm to confirm deletion.

Prefixes and suffixes can only be deleted if they have not been applied to a call number. The prefix or suffix must be removed from every call number it is in use with before the prefix or suffix can be deleted.
You may also need to log out and back in again for the deleted prefix or suffix to disappear as an option.