Bollinger Bands %RSI — Indicator by kinetix360

Hi All,
I am not a programmer, but I tried to buid BB% of RSI , base of John Bollinger' book. I cut and paste the function from LazyBear MFI/RSI BB indicator and original BB% scripts. Now, I need help for 2 things :

1. I already check by side with LazyBear indicator for the OB/OS, and all are good. One thing I don't understand is why we build the Basis for bb using "sma" ? any one can help me to understand with this?

2. I am happy with this, but I need to make the Source Price become customisable (close, hl/2, hlc/3, etc). and I don't know how to set it up. please help me with this.

Thank you.

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study(title = "Bollinger Bands %RSI", shorttitle = "BB %RSI")

source = hlc3
length = input(14, minval=1), mult = input(2.0, minval=0.001, maxval=50)
HighlightBreaches=input(true, title="Highlight Oversold/Overbought?", type=bool)

//Define RSI
rsi_s = rsi(source, length)


// BB of RSI

basis = sma(rsi_s, length)
dev = mult * stdev(rsi_s, length)
upper = basis + dev
lower = basis - dev

bbr = (rsi_s - lower)/(upper - lower)
plot(bbr, color=teal)
band1 = hline(1, color=gray, linestyle=dashed)
band0 = hline(0, color=gray, linestyle=dashed)
fill(band1, band0, color=teal)

//p1 = plot(upper, color=blue)
//p2 = plot(lower, color=blue)
//fill(p1,p2, blue)

b_color = (rsi_s > upper) ? red : (rsi_s < lower) ? green : na
bgcolor(HighlightBreaches ? b_color : na)