You may only be losing a few customers, but if they're high-paying customers, your business has a problem. SaaS businesses therefore look at revenue churn rate, which treats lost revenue independently of the customer base that it's made up of.
It's entirely possible that a SaaS business will lose a high number of customers but not much revenue in a given period, which is why you should check both periodically.
For instance, you might lose five customers, but four of them pay you virtually nothing. This could lead to a worryingly-high user churn rate, but the revenue impact could be minimal. Comparing user and revenue churn gives you a better idea of the composition of your customer base and the seriousness of your losses.
A company that has low or zero revenue churn but some user churn could be shedding less valuable customers while retaining high-paying ones.
Even better, a business that has negative revenue churn is growing billing from existing accounts (not new customers), which is an encouraging sign.
Lots has been written about revenue churn and revenue retention. Some of our favorite pieces include: