Would Like to Clarify Ops of some Functions in the Calculations Command in Ps

I think that it is easier to learn calculations by first learning to use the Apply Image command, which simplifies and clarifies the process and concepts.

• Does the Blending Mode in the Calcs Cmd apply to the doc./channel selected for Source 2?

By @Fotomaker01RA

The blending mode applies to the result of the operation between source 1 and source 2. If you swap the source around with the same blending mode etc., you get the same result (swapping source 1 for source 2 and source 2 for source 1). From the behaviour of the related Apply Image command, I would say that it's calculated from source 1.

• What does Scale do for me - I know it runs from 1-2 but what does it help me achieve (for making masks/selections) if I move the numbers along that spectrum?

Offhand, the most common example of using the scale and offset feature for Add or Subtract blending that I can think of is in Frequency Separation.

When creating the high-frequency layer, working in 16 bpc mode for a mathematically lossless conversion, a scale of 2 is used in the Add blend, darkening the result to create a "neutral" mid-grey result for blending the resulting layer in Linear Light blend mode.

If working in 8 bpc for a mostly visually lossless conversion,  a scale of 2 and a rounded offset of 128 (mid-grey) is often used and set to Subtract blend mode.

• What is Invert inverting? Again, is it preferable to only invert either Source 1 or Source 2? Is Invert only useful when using a Subtract blending mode?

Invert simply provides on-the-fly flipping of the values of the pixels in either source channel. White becomes black, black becomes white, 75% becomes 25% and 50% remains the same. It is useful when it is useful, as it all depends on what you are doing. That's not intended as a glib reply!  :]

• What does clicking the Mask option do? Why wouldn’t we just return whatever we come up with in the Calcs Cmd to an alpha channel then make it usable as a mask in the Ps layers stack? Does Mask achieve the same thing, but more directly? If yes, is any flexibility lost wrt how the mask can be used vs going the alpha channel to mask route?

Mask gives the user the flexibility to use a primary or alpha channel from any supported open document during the creation of the channel (or selection or doc). Black hides, white reveals. This is a useful and powerful feature, with or without Layers. Before Photoshop 3.0 in 1994, there were no layers, only a temporary floating selection with pixel content that was merged afterwards! The Apply Image and Calculations commands pre-date Layers.