Dynamic Type Model|OPC UA Standard|Forum|OPC Foundation

Adding types while is server is running is no problem.

Deleting types can be problematic but it is not explicitly prohibited for leaf types.

Modifying types or deleting supertypes changes the type contract which is a breaking change and not allowed.

You also should never re-use a NodeId if a type is deleted and re-added.

Clients may cache NodeIds (particularly type NodeIds) between sessions. If you re-use NodeIds clients will be accessing a different NodeId when they reconnect and may not realize it. This could lead to some unexpected behaviour and very hard to find bugs.

So re-using NodeIds should be avoided but not prohibited. In some cases, NodeIds come from an underlying system so the UA server has no control over what the underlying system does.

A type is a API defined by the hierarchy of nodes under the type. Adding or removing mandatory components in the hierarchy or changing the node class, type definition, datatype or method argument of any components would break the API contract. It is allowed if done when all Sessions are closed but should be avoided since clients can be configured with the results a previous session where they browsed the tree and cached the results.

That said, it is not prohibited because the UA server could be building a type tree from an underlying system and the UA server has no control over that system nor any knowledge changes to that system over time (i.e. the type may have changed but the UA server can't know that it did).