What do the small yellow squares at the end of lines and arcs in a sketch mean

[KarolR]

in the connection between 2 lines, or line and arc, there are yellow squares, but no on point constraints.

What do they mean?

I copied a sketch to another sketch, added a line and the yellow square didn't appear.

I couldn't add it even with a new line, and couldn't extrude the sketch.

[JD Mather]

Coincident Constraint.

Somewhere there is a option to show them as the yellow squares you see or as the usual constraint glyphs.

or you can turn them off Hide All Constraints.