By Garry Stockton
It can sometimes be a bit confusing to understand what Attachment and Overlay mean in Reference Types, especially for new users. Hopefully, this will help you understand the difference between Overlay and Attachments in Revit.
When linking a Revit or IFC model into your project, you have the option to change the Reference Type to either Overlay or Attachment. So, what does it mean? If you select one or the other, it makes no visible change to your model, so it is easy to ignore, but the Reference Type is actually very important if your Host Revit model gets linked into another model. Your chosen reference type will then determine how that link behaves in the new model. In this blog post, I will explain the difference between Attachment vs Overlay.
Overlay vs Attachment in Revit.
So, choosing the reference type of a link determines how Revit will regard that link when your host model is linked into another model.
For example, let us say we are working in Project B and we Link in Project A. If Project A’s reference Type is set to Attachment, when we Link Project B into Project C, Projects B and A will be visible. Basically, Project A will get attached or nested into Project B. See below:
For the same scenario, if Project A’s reference Type is set to Overlay, then when Project B gets linked into Project C, only Project B gets linked in and Project A gets ignored. Below:
From a structural perspective, this is important if you are working with an Architects model who may have a lot of Linked models (such as Site models, repeating floor plans etc.) If the Reference Type of those Links are set to overlay (in the Architects model) then they will not show up in your structural model.
Similarly, if you are sending your Structural model back to the architect to be linked into their model, you may want to set their Linked models reference type to Overlay so it does not get re-linked as well.