Is this the same as dimethicone?

Same family, different form. Dimethicone is a fluid with a chain backbone; this is a cross-linked resin made as solid spheres. They feel different and do different jobs.

It is an insoluble particle that stays on the surface, and it is not an oil. Whether a product breaks you out is usually a question about the rest of the formula.

It blurs more than it mattifies. It scatters light rather than absorbing oil, so shine can come back while the softened look is still there. Formulas that need both usually carry an absorbent powder as well.

Is it a microplastic?

It is a solid, insoluble synthetic polymer particle, which is the category several markets have written restrictions around. Whether it is in scope where you are is a question for the current local rule — the definitions and the deadlines differ and are still moving.

Does it do anything for my skin?

Not in the way an active does. It changes how skin looks and feels while it is on, and that is the whole of the claim.

Polymethylsilsesquioxane — Ingredient Guide

How it works

Silicones come in more forms than the oily fluids the word usually brings to mind. This one is a resin: the silicon–oxygen backbone is cross-linked in three directions rather than running as a chain, and the material is manufactured as small, hard, near-perfect spheres instead of as a liquid.

Two consequences follow, and both are mechanical. A sphere scatters light evenly in every direction, which is the most efficient shape there is for filling in the shadow that makes a line or a pore visible. A hard sphere also rolls, so a formula carrying it feels dry and powdery where a silicone fluid would feel slippery. Neither behaviour asks anything of the skin.

The evidence

The optical and sensory effects are immediate — you can see and feel them on application, which also means there is nothing to wait for and nothing that accumulates. The distinction worth holding on to is between blurring and mattifying. These spheres blur by scattering light, and they take up very little oil compared with a starch or a clay, so a product built on them can soften the look of texture while shine comes back on its own schedule. Formulas often carry both kinds of powder for exactly that reason. Size and uniformity matter more than chemistry here: a batch of spheres close to one diameter scatters light predictably, which is what a formulator is buying, and that specification belongs to the supplier rather than to the label.

What it does not do is form a film. Cross-linked resin particles stay particles; the continuous layer people associate with silicones is the work of the fluid ones, and this is not one of those.

Suitability

Not an active; no skin type needs it and none needs to avoid it
Suits anyone who dislikes a slippery or greasy finish, because the finish here is dry
Turns up in primers, mattifying moisturisers, sunscreens and anything sold on blurring
Insoluble and not absorbed: it stays where it is put and washes off

Concentration

No figure. The level is set by how much blur and how dry a finish is wanted, and past a point more of it simply reads as powdery or grey on skin. Nothing about skin sets a threshold.

Conflicts & combinations

Nothing conflicts with it chemically. The live question is regulatory rather than dermatological: solid, insoluble synthetic polymer particles are the category the microplastic rules in several markets are written around, and those rules are market-specific and still being revised. Whether this material is in scope where you live is worth reading in the current local rule rather than in anything a page can settle once.