SaaS metrics include some metrics that are based on total value of a customer and costs to acquire new customers.
When you have products like additional user seats, or add-on modules to products, these should not be included as distinct subscriber counts when calculating customer-centric SaaS metrics like LTV and CAC.
Just check the "Add-on product" box in the Metrics tab of a Product plan item, and that product's subscriber count will not be included as additional customers when tallying your total subscriber counts in SaaS metrics.
For example, if you spend $50,000 acquiring 50 new customers for your $1000/month product, and all of them purchase a $100/month security add-on module, you have spent $1000/customer acquiring 50 customers with a monthly ARPU of $1100, not $500/customer acquiring 100 customers with an ARPU of $550. Your LTV will also be adjusted accordingly when you designate certain products as add-on products.