Insubstantial portions can be reproduced without needing the permission of the copyright owner. The use of insubstanital portions are not restricted to a particular purpose however it is important to be aware that substantiality refers not just to the quantity of the portion but also to quality. This means that even if a portion is small if it is key or central to the work then it is likely to be considered substantial. For example a chorus of a song or a well known line from a movie may be considered substantial despite being a small portion of the overall work.
You must attribute the creator of the work even when using an insubstantial portion. This is a moral right.
If you wish to use more than an insubstantial portion of a work, depending on the purpose, you may be able to rely on one of the other exceptions in the Copyright Act. If no exceptions are available you can seek permission.