Add Multiplied Scalar as in the last image of the first post is a compact way of connecting an Add Scalar with a Multiply Scalar together in the way that's shown. That's all that is going on.

So if you have a value of 0.5 in the Main Input and 1 in Input 2, the maths is:

0.5 x 1 + 0.5 = 1

If the Main Input is -1 and Input 2 is 1:

-1 x 1 + -1 = -2

or with Main Input is -0.5 and Input 2 is 2:

-0.5 x 2 + -0.5 = -1.5

and so on.

If you add vectors or colour into the inputs, there is a conversion that happens to those three component data streams to make a scalar value with one component. I've tried to cover that in Add Scalar and you can find further information in the the wiki

here.

Masking (which may have been a descriptive error on my part in the initial post) is essentially multiplying one scalar value with another to hopefully produce an alpha mask of a range of 0 to 1. Black to white.

As you mentioned, you can put in other values which fall out of this range of 0 to 1 depending on the input values.