Enterprise customers which wish to add subdomains (or other similar sites) of a site they have already configured at Incapsula, have two general options:
1. If both sites/domains/subdomains are sitting on the same server/IP you can use the Incapsula CNAME given by the system for the initial site, and use it for them.
In this case, you will see aggregated statistics in the Incapsula dashboard for all domains configured behind the same CNAME under the initial site that is configured on Incapsula. Every site using the same CNAME will also be regarded with the same security settings set to the original one.
Using this method will not incur any extra charges, as this does not count as adding an extra site (except for data charges beyond the plan).
For more information: CNAME Reuse
2. If you wish to separate statistics and security settings for the added subdomain, then it should be added as an "extra site" to the account (by pressing the Add Site button on the sites page), and thus will be billed as an extra site according to the plan's details.