Dear Manasi,
Your client is asking to take registration under the BOCW policy, which means you have been awarded work of construction. The BOCW and The BOCW Cess are for safety and welfare measures for the construction workmen.
On the other hand, the WC policy covers liability under compensation to be paid to workmen in case of any accident resulting in the loss of working capacity or fatal, partial, or permanent disability.
I hope this explanation is sufficient to clarify your doubts.