Please don't shut your mind and blindly regurgitate answers. This is a major problem with Quickbooks and it is causing our company some major problems with inventory management. 

I'm going to restate the question referencing the "Build Assembly" screen. Go to a build assembly and populate it with a build and make the build a pending build with the build amount set to "1".

In the upper right hand corner of the build, it lists the:

quantity on hand

quantity reserved for other assemblies

quantity available

For my item it lists:

Quantity on Hand: 200

Quantity Reserved for Other Assemblies: 10

Quantity Available: 190

Now click Build and New and then navigate back to this build.

It now Lists 

Quantity on Hand: 200

Quantity Reserved for Other Assemblies: 11

Quantity Available: 189

So, effectively making a pending build of an assembly reduces the quantity available of that assembly by 1.