30% and 70% typically indicate oversold and overbought levels. They are not very reliable indicators for making buying and selling decisions.

It's well known that RSI rising above 50 indicates an up-trend, e.g. from Chartmill Above 50 the technical trend is positive, below 50 it is negative.

It should be remembered that Welles Wilder introduced the standard interpretation for the RSI back in 1978. That's over 40 years ago. In Technical Analysis History that is now in the Jurassic Age. A lot of work by lots of clever people have been done on the RSI since Welles Wilder introduced the concept. One of the best known is Constance Brown whose book is required reading for the CMT exam.

Constance Brown in :

Technical Analysis for the Trading Professional, Second Edition: Strategies and Techniques for Today’s Turbulent Global Financial Markets
makes the following comment:

Do not lose sight of the fact that the original works provided us with the foundation on which our industry is growing today. The important distinction is that early books on technical analysis will eventually be viewed as classics, but traders who fail to evolve beyond these original concepts face a far less pleasant fate: extinction.” – (Brown, 2012, 6)

Constance Brown suggested the following:

• Bull Markets: In a bull market, the RSI ranges between an overbought zone of 80 to 90 and an oversold zone/support of 40 to 50.
• Bear Markets: In a bear market, the RSI ranges between an oversold zone of 20 to 30 and an overbought zone/resistance of 55 to 65.

Statistical work on the RSI suggests traders ignore Welles Wilder levels for overbought and oversold (70 and 30), i.e., sell if RSI is above 70 and buy if RSI is below 30. Instead, adopt a counter-intuitive state of mind and buy if RSI is above 80 and sell if RSI is below 20. Mind-blowing, huh?