When you are changing a custom unit and replacing it with another unit, or combining two custom units together, it is important that you change the unit reference correctly so you don't lose the link to the usages of the original unit. If you delete units incorrectly, it can causes errors in different parts of your account because important information linking the original custom unit and its usages would be missing,
To make sure this doesn't happen, whenever you try to delete a custom unit you will be prompted to pick a new unit to "replace all references to" the original unit. This means that anywhere the original unit was used will be replaced by the new unit. Before you confirm, select the new unit that you want to use in place of the original one.
This also preserves the connection of custom units to conversions that use that custom unit. For example, say you notice that you have two different custom units for Basil, "bn" and "bunch", as well as a conversion defining that "1 bunch" = "2 oz". If you just delete "bn", every recipe that uses "bn" as the unit won't have a value. By switching the unit reference first, all the usages "bn" will be replaced by "bunch", so the associated "bunch" conversion will also apply to those units.