How will enabling retention affect the storage?

How will enabling retention affect the storage?

Generally, the user's allotted storage capacity is shared by the User's mailbox + eDiscovery + Backup.
For instance, a user is allotted an overall storage of 100 GB. This storage can be used for any Zoho Mail feature that requires data to be stored. Considering, 20 GB from the overall storage is utilized by the user's mailbox, another 20GB for backup and 15GB by retention, then the overall storage utilized would be 55GB. This means, the remaining 45GB of space is available which again can be used by mail, retention and backup.
Therefore, it is recommended to refine what you want to retain in archival and for how long depending on your organization's needs.
Besides the storage available in the base plan, Zoho provides an option to buy additional add-on storage units for an organization and assign it to any user as required.