Your a's — versions of a letter
Nobody writes the same letter identically twice. Your a comes out slightly different every time, and that variation is most of what makes handwriting look handwritten.
A font can hold several versions of a letter and move between them as you type, so the same letter does not stamp identically down the page.
Why it matters more than it sounds
Look at a word with a repeated letter — letters, coffee, all. In a plain font, both copies are pixel-identical. The eye catches that instantly, even without knowing why, and reads the text as typed rather than written.
Give the font two or three real forms of a letter and the same word stops repeating. It is the single biggest difference between a font that looks like your hand and one that looks like a font of your hand.
Where the versions come from
They come from you. When you write a letter more than once, those are genuine versions of it, not copies with wobble added. Inkling will never invent a letterform you did not write — your font contains your hand and nothing else.
The richest source is writing whole words and sentences, where the same letter naturally appears many times in different company. That is what Words is for.
Which tier uses them
Moving between versions as you type is part of Pro. A Free font holds one settled form per letter, so repeats stay identical — see Free and Pro and Repeated letters look identical.
On Pro, every version you keep rotates through your words. Nothing is held back by a limit or by a judgment of ours: a version leaves the rotation only when it carries a warning mark you have not answered, when you place it as a flourish, or when you rest it yourself. Rest a version from the Letters wall or its own page to keep it without using it; tap again to put it back. A character's own page.
Adding more
Write the letter again from the Letters room. Concentrate on the letters you type most — e, t, a, o, n, s, r repay it many times over.