Can you help me understand client requirements about Forcepoint Next Generation Firewall?

Hi all,

I just want to understand the below requirements about Forcepoint Next-Generation Firewall:

As per the below bullets, I am able to assume that my client requires two firewalls with the HA configuration. However, I don't understand about (N+1) clustering with up to 6 nodes in a cluster.

• The firewall shall be configured in active-active.
• Configuration and horizontal scaling with N+1
• Clustering with up to 6 nodes in the cluster to provide the control and redundancy.

Can anyone clarify those technical requirements for me? 

Thanks for your help!