Below are some principles used for variable symbols.  These have the disadvantage of making expressions much longer, but the cost is compensated for greater [clarity, consistency, error-checking,etc].  

Redundancy - easier to see that a mistake has been made, or that thinking is sloppy
	This is like human language.  For example, in the symbol "Ooca(POIp,t)" the "a" is redundant, as "O" only refers to an angle.  Therefore, a reader who sees "Ooc(POIp,t)" should be on the alert that I may have made a mistake with the symbol.

Sparseness -  helps to reduce errors from [missing,switched] letters of symbols.  Only a few of the possible combinations of [symbols,qualifiers] are actually used.

Uniqueness from symbol length	Often "copy&replace" operations are used to update or change symbol conventions.  Short symbols like "r" require much more attention and manual colow-level objective, not yet implemented, would be to implement a constant charater-length standard for variable symbols (eg 5 character), so that alignment of symbols in multi-line formulae, or sequences of equations, becomes "near-automatic".   However, this causes other problems. 

Context	While it is normal to pre-specify the context of derivations, by incorporating key context into the symbols themselves, it is easier to remember specifics that are important to derivations.

Common concepts	To reduce a proliferation of symbols that are hard to rememeber, a common "basic symbol" (i.e. first letter or so) is used as much as possible.  For example "r" is used to denote [length, vector, displacement vector, etc].

Capital letters for the basic variable symbol   -   To clearly distinguish the basic symbol of a variable from qualifiers, the basic symbols are only in CAPITAL letters.
	Also, there aren't enough letters in the alphabet to cover all basic sysmbols, so if a new symbol is needed, sometimes two or more capital letters are used to denote it.  For example (POI) dentotes "Point Of Interest".

Small letters for qualifiers -  To clearly distinguish the basic symbol of a variable from qualifiers, the qualifiers are only in small letters.

"Find-and-Replace-ability"  -   When changing symbols or correcting errors in formulae, having [long, distinct] symbols can greatly aid with cut&paste operations.  For example, if [r, roc, rocs] are used, then a mass " Find-and-Replace" operation on an entire file, or section of a file, for "r" becomes an intensely manual "r-by-r" operation.   The use of [rocv,rocs] is much easier.