The best way to learn about your brushes is to experiment, but meanwhile, here’s a handy guide to oil painting brushes.
Above is a sable flat brush by Rosemary & Company. It can put down a very smooth surface and offers a lot of control, but it doesn’t carry the quantity of paint that an equivalent bristle brush will. I do have many sable brushes, but I save them for thin work in the studio.
This is a Robert Simmons Signet flat brush. The paint it lays down is both rougher and more impasto than the sable.
Flat brushes make an immediate, energetic mark. They’re excellent for fast, powerful surface work, long sweeping strokes, and blocking in shapes.
Used on their sides, they also make great lines, far more evenly than a small round can do.
Two rounds of very different sizes. A round is a more lyrical brush than a flat, and is a classic tool for painterly surface marks. It can be used to make lines that vary from thin to thick. A pointed round is used for fine detail. Bristle rounds tend to lose their points very quickly, however.
A bright is a less-flexible version of a flat. It’s great for short, powerful strokes or situations where you want a lot of control.
A fan brush probably has no place in a plein air kit, but I carry one anyway. I use it for blending, as on the left, although some people like using it to make whacked out marks as on the right. The problem is, it can carry very little paint, so its marks tend to be either gooey, as above, or very abrupt.
In my studio, I just use a clapped out soft-haired brush to blend.