A question posted at AUGI started out simply by asking if this concept is the same as for AutoCAD. I replied, "Exactly the same concepts". Then another member expanded on my reply and it seemed to muddy the water for the person who wrote the original post because when they replied it sounded like it was still unclear. I wrote this longer reply and decided to put the explanation here too.
Choosing between attached or overlay is to determine how Revit should regard a link when the host file is linked into yet another file. For example, if File 1 is linked into File A and set to "attached", when File A is linked into File B, File 1 will also also be loaded into File B. If File 1 is changed, when its host File A is reloaded in their host File B, File 1 changes will also be seen, if its own changes have been saved before Reload is used. Both files are read independently during a Reload. You will only find the host File A listed in File B's Manage Links dialog however. Written another way, when a link is reloaded, it's attached nested links will also be reloaded.
When Overlay is chosen instead File 1 does not get loaded into File B. No circumstance will cause it to appear nor will we be aware of changes within the link.
We should use the "Attached" setting when we link a file into another file AND expect its own linked files to show up as if they are native elements in the file too. Changes in either the parent or child will appear when the parent file is reloaded into its host or if the host file is opened after changes have occurred in either.
In practice, if we create an architectural model (parent) that uses linked files (children) for repeating units they will need to be set to Attached if we want the units to appear when the "parent" architectural model is linked into a structural model. When we pass along our model to the structural engineer we will need to send both the "parent" architectural model and the "child" repeating units. When we link in their structural model we should use "Overlay" so Revit does not try to load their structural model when the structural engineer links our architectural model into their own model.
Clear as mud?
Choosing between attached or overlay is to determine how Revit should regard a link when the host file is linked into yet another file. For example, if File 1 is linked into File A and set to "attached", when File A is linked into File B, File 1 will also also be loaded into File B. If File 1 is changed, when its host File A is reloaded in their host File B, File 1 changes will also be seen, if its own changes have been saved before Reload is used. Both files are read independently during a Reload. You will only find the host File A listed in File B's Manage Links dialog however. Written another way, when a link is reloaded, it's attached nested links will also be reloaded.
When Overlay is chosen instead File 1 does not get loaded into File B. No circumstance will cause it to appear nor will we be aware of changes within the link.
We should use the "Attached" setting when we link a file into another file AND expect its own linked files to show up as if they are native elements in the file too. Changes in either the parent or child will appear when the parent file is reloaded into its host or if the host file is opened after changes have occurred in either.
In practice, if we create an architectural model (parent) that uses linked files (children) for repeating units they will need to be set to Attached if we want the units to appear when the "parent" architectural model is linked into a structural model. When we pass along our model to the structural engineer we will need to send both the "parent" architectural model and the "child" repeating units. When we link in their structural model we should use "Overlay" so Revit does not try to load their structural model when the structural engineer links our architectural model into their own model.
Clear as mud?
