1. FAST Content SSA
2. FAST Query SSA
The most confusing thing is we can create the content sources in both of these SSA's. The rule here is that we should be setting all the content sources other than People Search in the Content SSA (Connector SSA). So this SSA is primarily used for doing the crawling on various content Sources. FAST Query SSA also have a crawler but in this SSA we should set up content source only for the People Content.
So the FAST Query SSA primary role is to take the queries and pass it to the connector SSA which is responsible for indexing and crawling the content (again other then People Content source).
Also FAST Query SSA can be only associated to any SharePoint Web Application. The FAST Query SSA has a Deault Search Provider property which determines where the query should be routed. If we are using the FAST Content SSA the queries will be routed to this SSA. We can change this default search provider by using this PowerShell Script:
Set-SPEnterpriseSearchServiceApplication -Identity "FAST Query SSA" -DefaultSearchProvider "FAST Content SSA"