Fixed
fix pins a point to the plane. A profile whose shape and size are fully constrained can still slide and spin as a rigid body; anchoring one point removes those last degrees of freedom. It is the constraint that turns "the right shape somewhere" into "the right shape here".
Coincident
coincident makes two points the same point, or holds a point on an entity. It is the constraint that closes profiles: consecutive lines and arcs are separate entities until their ends are made coincident.
Horizontal
horizontal aligns with the sketch's X axis: a line becomes horizontal, or two or more points are brought to the same height.
Vertical
vertical aligns with the sketch's Y axis: a line becomes vertical, or two or more points are stacked on the same x.
Parallel
parallel gives two or more lines the same direction. The first line is the reference; every other line turns to match it, and turns again whenever the reference turns.
Perpendicular
perpendicular holds two lines at 90° to each other. Unlike horizontal + vertical, it says nothing about the sketch axes — tilt one line and the other follows, still square.
Symmetric
symmetric mirrors two points across a line: they end up the same distance from it on opposite sides, on a line perpendicular to it. It is how one dimension drives both halves of a symmetric profile.
Midpoint
midpoint holds a point at the middle of a line, or halfway between two other points. A hole that must stay centred on a bar however long the bar becomes is a midpoint, not a distance.
Tangent
tangent makes a line and a circle/arc, or two circles/arcs, touch smoothly — no corner where they meet. Together with coincident at the junction it produces the kink-free outlines of rounded profiles, belts and cams.
Equal
equal gives two or more lines the same length, or two or more circles/arcs the same radius. One dimension on the first entity then sizes all of them.
Concentric
concentric gives two circles or arcs the same center — a bore and its counterbore, a rim around a hub, a washer.
Collinear
collinear puts one line on the infinite extension of another. The two segments stay separate entities — they can have a gap, or overlap — but they share one line.