What this means in practice
When planning capacity, account for these uses together rather than assuming each feature has a separate allowance.
Practical guidance
Object storage is a shared pool for files and objects used by several services. Saved files, mail attachments, hosted content, and contact photos all draw from the same allowance. There is not a separate hidden quota for each of those features, so a change in one area can affect remaining capacity for another.
When storage approaches its limit, inspect the objects you still need and identify large or obsolete items across services rather than looking only at one feature. Deleting a file may free room that can be used by mail or hosting as well. If you estimate future needs, include all of these uses together. This shared view makes the storage total easier to reconcile and prevents surprises when one service adds objects to the same pool.