* The unit size for `VirtualMachine.disk` and `VirtualDisk.size` has been changed from 1 gigabyte to 1 megabyte. Existing values will be adjusted automatically during the upgrade process.
* The `min_vid` and `max_vid` fields on the VLAN group model have been replaced with `vid_ranges`, an array of starting and ending VLAN ID pairs.
* The five individual event type fields on the EventRule model have been replaced by a single `event_types` array field, which lists applicable event types by name.
* The `validate()` method on CustomValidator subclasses now **must** accept the request argument (deprecated in v4.0 by [#14279](https://github.com/netbox-community/netbox/issues/14279/)).
#### Circuit Groups ([#7025](https://github.com/netbox-community/netbox/issues/7025))
Circuits can now be assigned to groups for administrative purposes. Each circuit may be assigned to multiple groups, and each assignment may optionally indicate a priority (primary, secondary, or tertiary).
#### VLAN Group ID Ranges ([#9627](https://github.com/netbox-community/netbox/issues/9627))
The VLAN group model has been enhanced to support multiple VLAN ID (VID) ranges, whereas previously it could track only a single beginning and ending VID pair. VID ranges are stored as an array of beginning and ending (inclusive) integer pairs, e.g. `1-100,1000-1999`.
Module bays can now be added to modules to effect a hierarchical arrangement of submodules within a device. A module installed within a device's module bay may itself have module bays into which child modules may be installed.
A new rack type model has been introduced, which functions similarly to device types. Users can now define a common make and model of equipment rack, the attributes of which are automatically populated when creating a new rack of that type. Backward compatibility for racks with individually defined characteristics is fully retained.
The NetBox UI now integrates directly with the canonical [plugins catalog](https://netboxlabs.com/netbox-plugins/) hosted by [NetBox Labs](https://netboxlabs.com). Users can now explore available plugins and check for newer releases natively within the NetBox user interface.
NetBox now includes a user notification system. Users can subscribe to individual objects and be alerted to changes within the web interface. Additionally, event rules can be created to trigger notifications for specific users and/or groups. Plugins can also employ this notification system for their own purposes.
* [#14656](https://github.com/netbox-community/netbox/issues/14656) - Dynamically render the custom field edit form depending on the selected field type
* [#15156](https://github.com/netbox-community/netbox/issues/15156) - Add `display_url` field to all REST API serializers, which links to the corresponding UI view for an object
* [#17288](https://github.com/netbox-community/netbox/issues/17288) - Introduce a configurable limit on the number of aliases within a GraphQL API request
* [#17289](https://github.com/netbox-community/netbox/issues/17289) - Enforce a standard policy for local passwords by default
* [#17318](https://github.com/netbox-community/netbox/issues/17318) - Include the assigned provider in nested API representation of circuits
* [#16776](https://github.com/netbox-community/netbox/issues/16776) - Add an `alerts()` method to `PluginTemplateExtension` for embedding important information on object views
* [#16886](https://github.com/netbox-community/netbox/issues/16886) - Introduce a mechanism for plugins to register custom event types (for use with user notifications)
* [#14861](https://github.com/netbox-community/netbox/issues/14861) - The URL path for UI views concerning virtual disks has been standardized to `/virtualization/virtual-disks/`
