They are more different than their names sound. RS, "Relative Strength"... is the measure of one issue vs. another. Example... if the Nas goes up 3% while the SP goes up 2%, the Nas has shown greater "relative strength" vs the SP. RSI, "Relative Strength Index"... is a technical tool that shows where an issue is currently vs. a certain number of recent days... 14 days/bars is the stock default. Think of it as a form of "overbought/oversold oscillator". Similar to other "range indicators".
My interpretation of discretionary. Firstly, I'm a rule based, algo, fundamental, technical, discretionary trader, I use all five simultaneously. The discretionary part; although my rules and algos may direct me, ultimately I decide from experience and market conditions, whether to trade and what to trade. Computers and rules are fine because they funnel you quickly to a decision, but they are not enough to capture all the information floating about. For some, they will trade straight off a computer based signal, for me, accuracy is more important than quantity, so my brain brings in checks and balances.
To me a "discretionary trading" is making a trading decision manually vs. programming a trading plan/system/decisions in various scenarios into a software program and running the program to trade automatically. You trade at your discretion and not by a set software.