Other candidates for external definitions are your own unique, proprietary parts that you use in more than one assembly. Our sample arbor press, for example, is made in a variety of throat sizes and shut heights. As indicated in the assembly browser, most parts appear in other assemblies and, hence, are external. The three parts unique to this model are internal. You can make a local part active at any time and edit it within the assembly.
All instances update automatically.
An external definition is like an xref in that you can edit it only by opening its own file. A common wish list request is for the ability to edit an externally referenced definition from within the current assembly. Be careful what you wish for. You might get it. Go check three paragraphs back for my thoughts on read- only directories.