Any Nimble snapshot that taken as part of a schedule is by design non-writable. This is because your schedule is part of your backup strategy with RPO/RTO assigned. If the snapshot was writable then it may be possible to change (or even delete) data thats in your backup - thus invalidating your RPO/RTO.
The only way to write to this snapshot is to clone the snap into a writeable child volume, meaning any delta changes made to that data will not ruin your backup RPO/RTO.
A manual snapshot (ie a snapshot manually initiated) CAN be writable, as that is a user-requested item and not part of a backup policy.
Note: a non-writable snapshot will cause problems if attempted to be mounted by Windows or VMware - so the best practice here is to always clone to a new volume should a snapshot need to be accessed.
When a snapshot is offline it means it is not visible and cannot be accessed by a host. When it's online it means it's visible and can be mounted by it's associated hosts as referenced in it's ACL.
Hope that's cleared it up, please let me know if you have any further questions.