A picklist keeps a business classification within a defined vocabulary. A multi-select picklist permits several choices, but its reporting and formula behavior differs from a single choice. Don't use one merely to avoid deciding what the field means.
A dependent picklist restricts its available values using a controlling field on the same record. The dependency matrix describes allowed pairs. Creating both fields without a matrix doesn't establish the restriction. Record types can narrow values further, so a value present in the global definition may still be unavailable in one process. Include the blank and mismatched cases when testing a dependency. A value from the wrong parent category must not slip through because it looks reasonable on its own.