Learn English – Can we use ‘availabilities’ as the plural of ‘availability’

There is little to memorize about this topic; your intuition should be enough. But do memorize these two things:

1. Countable nouns can be changed into uncountable nouns:

   • She fed the baby a teaspoon of apple.
   • Blend the slices of mango in a mixer to turn them into juice.

   Uncountable nouns can be changed into countable nouns:

   • There are several new butters being produced without milk.
   • Can I have two sugars, please?

2. You can have a noun being treated as both collective and singular in the same sentence (George Yule, Explaining English Grammar. Oxford, 1998):

   • The family HAS decided THEY can't afford a big wedding.
   • The audience WAS cheering and clapping THEIR hands.

In ordinary usage, nouns like "milk" and "water" are uncountable.  There are times, however, when such words do have a countable sense.  For example, cows produce a different milk than goats.  If I want to compare those two milks, then I'm using the word "milk" in a countable sense and phrases like "a different milk" and "those two milks" are natural and sensible. 

In financial contexts, it is natural and sensible to talk about different countable monies.  The USD and the Euro are two different monies.  Taxable income and tax-free income are two different monies. 

Strictly speaking, it isn't necessary to force a countable sense onto an uncountable noun.  It is just as natural and sensible to talk about "two types of milk" or "two kinds of money", which allows "milk" and "money" to retain their ordinary, uncountable meanings.  Your cited definition includes the phrase "as used in financial contexts".  A better phrasing might be "as used in financial jargon".