Limitations

Designs is in preview. These are the known gaps, and what to do about each until it's closed.

What Designs manages

Designs works from the data model of NetBox itself, so it manages the objects NetBox ships, and only those.

Custom fields aren't supported yet. A design that sets a custom field's value isn't refused, but the value isn't written. Plans neither read nor change the custom field values objects already have.
Custom objects aren't supported. The object types you define in NetBox yourself aren't part of NetBox's own data model, so a design can't describe them.
Plugins' models aren't supported. A design can't describe what a plugin adds, such as netbox-branching's own objects. Planning into a branch still works: see Plan into a branch.

A design can't be renamed. Its name is how other designs and its deployments refer to it. To put a design under another name, save it as a new design and delete the old one once nothing uses it. See Edit a design.
A design that other designs include can't be deleted. It can be edited.
A deployment can't be renamed.

Variables have no types. A variable is text until a field uses it, so a mistyped number or address is caught at plan time, by the field, rather than when you enter it. Typed variables are planned for general availability.
A loop's bounds can't come from a variable. A design that loops over the same count in several places, as the compute rack does for its servers and their cables, repeats the count in each.
Designs don't allocate. No function picks the next free prefix, address or ASN: a design states the ones it wants.
Some Device Type Library components aren't brought in yet: power outlets, front and rear ports, and a module type's components other than its interfaces. A cable that ends on one of these declares the port in the design, under the device, so there is a port for it to end on.
Device types from NetBox Data Exchange are planned for general availability.

Two deployments that set the same field to different values overwrite each other. Each plan writes its own value back, and no plan says another deployment sets that field. Keep a field in one deployment.
A deployment planned into a NetBox branch is that branch's. After you merge the branch, plan the design against main to manage its objects there. See Plan into a branch.

The first plan against a NetBox reads all of it, and can take a minute on a large one. Later plans read only what changed.