The Constructor places items or fluids from the storage system in the world.
No other blocks can be in front of the Constructor (except fluids) and it should be possible for the block to be placed (for example: saplings need dirt).
The Constructor can also drop items in the world instead of placing actual blocks. When provided with a Stack Upgrade, it'll drop stacks of items at a time.
My Constructor isn't doing anything!
FTB Utilities' protection feature blocks the Refined Storage fake player from placing blocks.
Run this command to bypass this protection:
/ftb team config ftbu.fake_players true