The assembly database has a Standard folder and a Flat Rate folder.
The labor unit (this is how long it will take to lay-out and install the assembly) assigned to the assemblies in the standard folder is the sum of the labor units for all of the items within the assembly.
Flat Rate assemblies:
Flat Rate assemblies are typically used for service calls or small remodel jobs where you will show your customer what you're charging for each assembly. This allows you to show your customer not only the total price for the entire project, but also what you're charging for each assembly (or task). This will allow your customer to understand where the larger total price is coming from.
The labor unit assigned to the assemblies in the flat rate folder is not the sum of the labor units for all of the items within the assembly.
The labor unit assigned to flat rate assemblies is an overridden labor unit. There is a check-box in the assembly maintenance screen that can be checked to override the calculated labor unit. The overridden labor unit reflects a realistic amount of time that it should take to install the assembly on a service call. It reflects a worst case scenario amount of time. However, it's not large enough to make your price to high where you wouldn't get the job. It simply accounts for the "unknown" factor.
In summary, the standard assemblies are used for new construction whereas flat rate assemblies are used for service calls or any type of work where you need to add a little time buffer.