Consider this a shortstop between you and inconsistent data. It can really help to “lock down” fields like Customer Type and Sales Rep to prevent bad entries.
Bring up the picklist by clicking into the field and then clicking the grey arrow to the right.
From there, click the “Setup” button.
What we are concerned with is “Allow Blank Input” and “Force Valid Input”. Forcing a valid input requires the end-user to use a value from the picklist.
But beware, disallowing Blank Input will prevent the end-user from “getting out” of the field until they choose something valid. So make sure that all appropriate picklist choices are available!