Manual versus filter-based opportunity territory assignment
Key Takeaway
Give an opportunity a territory in one of two ways. Manually, Salesforce admins and sales managers pick one in the "Territory" field on Opportunity page layouts. Automatically, filter-based opportunity territory assignment runs the filter logic in an Apex class.
In Sales Territories, you assign opportunities to territories so the right reps and managers work each deal. You can assign the territory by hand on the opportunity. You can also turn on filter-based opportunity territory assignment so an Apex class assigns the territory for you. The two methods differ in who or what makes the choice and in when the assignment happens.
What a territory on an opportunity does
Assigning an opportunity to a territory enforces the principle of least privilege for the reps and managers in that territory. Sales Territories makes sure the intended reps can work their deals and territory managers can access their reps' opportunities.
To let users see which territory an opportunity has, add the "Territory Name" and "Territory Description" fields to Opportunity list views and reports.