what does effect the using set target library subset?

[u24c02]

Hi.

I have found like commends of DC.

set_target_library_subset -top -dont_use "*AT40"
compile_ultra -gate_clock ... -scan
...
remove_target_library_subset -top
compile_ultra -gate_clock ... -scan -inc -retime

I don't know what does these mean

 

[FvM]

Already reviewed the DC command reference manual?

 

[u24c02]

Sure. But I don't understand what they are effect to output netlist. Of course, I know what does mean each of commend. But you can see the above sequences if I use above total sequence what does benefit in there ?this is what I want to know.
Why do this for what? Does anyone know what does that meaning?

Especially, there are set_target... and remove target... commends, why these are inserted in there? Is there any specially reason ? Just for help to understanding to you,I just let you know what I just trying to implement as bottom-up way and this commend just in sub block not top block.