Many commands require or optionally use specifications of points or vectors in 3D coordinate space.
In general, a point-spec can be:
In general, a vector-spec can be:
For rotations, the order of the two atoms defines a handedness, and right-handed rotations are positive. If a center point is not specified separately, it will lie on the atom-atom axis. If a center is specified, only the direction of the axis will be used. The first model in the axis atom-spec will be used for reference frame pinning unless a center point atom-spec or a coordinateSystem is also given.
The default coordinateSystem for interpreting vector specifications varies by command, and could be that of a specific data model or just screen coordinates (X horizontal, Y vertical, Z in/out of screen).
See also: command usage conventions