What this means in practice
It is a common pool rather than a separate top-up for each database, so consider both together when diagnosing an overage or estimating future storage needs.
Practical guidance
Additional database capacity is purchased per gigabyte for 100 credits per GB per month, and the purchased capacity is shared by NoSQL and SQL. It is one pool, not a separate expansion for each database. Consequently, growth in either database contributes to the same total and can use capacity that the other database might otherwise consume.
Review the usage of both databases together before buying more capacity. Identify whether growth comes from an expected workload, retained data, or records that can be removed. If you decide to expand, account for the recurring monthly credit cost and check the combined pool afterward. This makes it easier to forecast the capacity required for a project and prevents treating a top-up as exclusive to whichever database prompted the purchase.