It used to be that if you were using core AutoCAD, and had a file from someone using a vertical product, you needed to install an object enabler to be able to see those objects. For some of the verticals, that is still the case. The link below will show which verticals need to be loaded separately:
Nowadays, most of the enablers are built into the product so you don't need to worry about it. The objects just show up without any action needed. However, there is still a scenario where you may not be able to see objects from a vertical product in core AutoCAD. For instance, let's say someone is using AutoCAD Architecture 2013, and they send one of their files to someone using Core AutoCAD 2012. If the AutoCAD user opens that file, they will not see any of those ACA specific objects, even though the enabler is built into the product. The reason for this is file format. Since the 2013 products started a new file format, there are a few steps needed to get a user on an earlier version to be able to see vertical specific objects. The steps to do this are below.
1) Open the file in its native program. So if the obejcts are ACA 2013 objects, then open the DWG in that program.
2) Set the PROXYGRAPHICS variable to 1.
3) Do a SAVEAS, and make sure to choose 2010 as the file format to save it in.
If a user using Core AutoCAD 2012 then opens that file, the objects will show up correctly.