With the Distributor Assignment feature enabled, Partners with Distributors assigned to them will only be able to select those Distributors from the Distributor picklist when registering Deals.  Partners without assigned Distributors will not see the Distributor picklist when submitting Deals. 

Also, Distributors will only be able to register Deals and/or track the progress of the Deals associated with the Partners they are assigned to. 

To enable the Distributor Assignment feature, go to Settings > Distributor Module Settings > enable the "Partners can only select from their assigned Distributors" option > Save. 

Distributor Assignment Settings 

How do I assign Distributors to Partners?  

You can assign Distributors to Active Partners (only) from the Distributor and Partner Management lister view pages. 

Go to Distributor Management > select the checkboxes next to the Partners that you want to assign a Distributor to > Actions > Assign to Partner > select the appropriate Partner from the picklist > Save.  

Distributor Lister View - Assign to Partner (bulk)

Assign Distributor to Partner Window 

Go to Partner Management > select the checkbox next to the Partner(s) that you want to assign a Distributor to > Actions > Assign to Distributor > select the appropriate Distributor from the dropdown > Save. 

Partner Lister View - Assign Distributor (bulk)