Hello,
Basically an instance is a variation of the generic.
e.g. if you had an Steel Rod, there woudl be two dimensions length and diameter. Having instances enables you to quickly produce similar models and drawings of this rod.
Your generic may be 6 dia x 10 long, your first instance may be 4 dia and 12 long.
In assemblies you can create various assemblies using these instances quickly.
e.g. If you have three models each with two instances, tere are a number of alternative assemblies available to you.
This is just a quick explanation, if you need further explanation with reference to your problem, please give more information.
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Hope this helps.
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maybe only a drafter
but the best user at this company!