Expense rules allow you to have automatic allocation based on the merchant name of the expense.
Setting Up Your Rules
To create an expense rule, go to the user icon in the upper left hand corner of your screen > Account Settings > Expense Rules to add or update.
If you'd like to apply the rule to all expenses, rather than just one merchant, then simply enter a period [.] and nothing else into the When the merchant name contains: field.
In order to delete a rule that you have created, click the trash can icon in the upper right hand corner of the rule:
Create a Rule While Editing
You can also create expense rules while you are editing a given expense. To do this, simply check the box for "Create a rule based on your changes" at the time of editing.
Expensify also has the concept of implicit categories (think "sticky" categories). Each time you categorize a merchant, the system will remember what you chose and will automatically apply that same category the next time that same merchant comes up again.
Implicit rules will always be trumped by any expense rules you have set up like above.
For more information about Implicit Categorization, visit our article regarding categories.