What is it you want to know though? from that question its kind of asking for a breakdown on the types of symbols and what they mean individually and what they represent as a group.

But tbh mate if you want it on 6 lines your not going to be able to get anywhere near to describing all of the symbols used on engineering drawings.

If I were you i'd take the subjects out of the keynote...say electrical...get a few main electrical symbols, describe what they mean individually and then write a line about the other types of symbols within the group (electrical)...say for example' other types of symbols in this group can show polarity, No of Phases, phase angles, resistance etc...'

Try this site....looks to have everyhting you need under one roof...in the design/drawing section (middle of page)http://www.roymech.co.uk/

Hope this helps mate....what course you doing?