How to use company chops?
The usual ways of using chops in Hong Kong are :

Signature chop - usually rectangular in shape.
Together with signature of an authorized officer, most contract and bank transaction are regarded as valid.

Round chop - usual small in size and no place for signature.
This is for daily casual use to acknowledge receipt of document from courier company. This chop is not for serious event.

Common seal - usually stainless steel.
Only documents required to be signed in SEAL need to be chopped. Example of such documents including transfer of property.

In China, the official company chop is regarded as valid for contract even without signature.
There is a big difference.