Like StochRSI, MFI shows 2 indications. Strength & overbought/oversold, because it graphs like an oscilator. It mainly relies on RSI for it's basis.
MFI uses volume with RSI to determine if more money is coming into a stock, then going out. And because the consensus is, price follows volume. You can judge entry/exit with the RSI position on the graph. MFI above 80 signals entry and below 20 exit.
While StochRSI signals Be In the stock or Be Out of the stock. MFI signals get In or Out.
It has a different focus then OBV, Acc/Dist, or CMF. While being in the volume group, because it uses volume in it's formula. It is mainly used for decision making, rather then support.
Like the Aroon, I disagree with where the books place this indicator. I would put it in the momo group, where you find other decision making indicators. They have the aroon in the momo group for decisions and I would place that in the trend group.
RECAP, MFI determines entry exit, or positive negative momentum. It's name and placement in the volume group is misleading. IMO