By pressing the Constraint button on the toolbar the following pops up with different icons displayed depending on the geometry selected. Once icons gray icons indicate the constraints already placed on the sketch.

The following table lists all the constraint options and a description of each one.

FixedMakes location of geometry fixed. Lengths are unconstrained
Fully FixedMakes location and lengths of geometry fixed
HorizontalMakes line parallel to sketch's X-axis
VerticalMakes line parallel to sketch's Y-axis
Constant LengthMakes line have fixed length
Constant AngleMakes line have fixed angle relative to sketch coordinate system
CollinearMakes two line segments lie on same axis
ParallelMakes two lines parallel to each other
PerpendicularMakes two lines perpendicular to each other
Equal LengthMakes two line segments have the same length
Point on CurveMakes a point lie on a curve or line
MidpointMakes a point lie at the midpoint of a line, not necessarily lie on the line
ConcentricMakes two circles/arcs share the same center point
Equal RadiusMakes two circles/arcs have the same radius
TangentMakes a circle/arc tangent to a line,or other circle/arc