RichQY - to answer your questions. I included images of what each item is.
Fitting kit - Stand alone method to installing a gauge.
Indicator - Basically a shift light.
Visor - allows the gauges to become more visible.
The three items are basically accessories. You do not need both the gauge pod and a fitting kit. You do need either of the gauge packages, and than a way to mount them. The way you want to mount them is really your choice.
And to those wondering if the defi tripple gauge pod fits STi and '04 WRX, the answer to that question is yes.
Also on a side note, the tripple gauge pod comes unpainted.