A garment the exact size of the body cannot be put on, let alone moved in. Ease is the extra width added to the body measurement.
Two kinds of ease
Wearing ease lets you breathe, sit and reach. Even a close-fitting shirt needs a few centimetres round the chest.
Design ease is extra width chosen for the look: a boxy jacket, a loose coat, an oversized T-shirt.
In these drafts the tape measures only the body, and the ease is added in the formula: chest + chest ease gives the finished chest. Changing the ease changes the style, not the measurement.
Negative ease. Swimwear, leggings and briefs are drafted smaller than the body. The fabric stretches to fit, so the less it stretches, the less you take off.
How much ease the blocks use
Chest or bust ease, all round, as the blocks start. They are starting points; change them to taste.
Round the body, ease is usually shared equally between the back and the front, so the side seam stays at the side. The armhole is another matter: it is drawn a few centimetres below the armpit, more in a jacket than in a shirt and more again in a coat, so the arm can move and the garment can go over other clothes.