Can formulas be called form functions or global variables?

Which kind of formulas are you talking about? Do you mean for calculating a field's value?
If so, of course it can. You just need to place them in a function and then call that function from the field's Calculate event.
For example, let's say you have a document-level folder that sums up 20 fields:

function sum20Fields() {
var total = 0;
for (i=0; i<20; i++) {
total+=this.getField("Field"+i).value;
}
return total;
}

Then you can call this function from the field's Calculate event like so:

event.value = sum20Fields();

Yes, for calculating a field's value, to be called in another formula inside a custom calculation, which you answered.

What reference can I look to for a detailed explanation of the elements of this portion of your sample formula:

for (i=0; i<20; i++)

That's general JS syntax. You can read the reference for it here, for example:
https://developer.mozilla.org/en/JavaScript/Reference
Or follow this tutorial:
http://w3schools.com/js/default.asp

Functions can be placed in any action on a form, at the document level of a form, or in an application level folder. Depending upon where the JS code is located and written will determine the scope of the function.

If you need more information about Acrobat JavaScript, access the free documentation from Adobe's developer site. More information about JavaScript is available through the Mozilla Developer site.