Drop-downs are used to encourage (or enforce) consistency in data entry.  There are three main ways in which drop-downs are populated:

Entry of a list of valid values as a validation rule (Access) or a Check Constraint (SQL Server).

Use of a Foreign Key to a lookup list

Use of the zRef_Drop_Downs table.

The zRef_Drop_Downs table is the most versitile for average users.  A user simply needs to right click on a value in ESdat and select "Add to zRef_Drop_Downs" and a drop-down for this table/column is created.