When transactions are initially passed into MX from Internet banking, categories are assigned based on complex business logic. Some transactions however will end up being “uncategorized” because the business logic didn’t find the key data values it’s looking for.  Categories can be updated at any time.  Once the category has been modified, you have the option of applying that category to future transactions matching that same logic, or just to that one transaction.
Categorization is a critical component to the accuracy of all features within MX so we recommend focusing on refining initially, and ongoing as changes in your financial situations occur.