Now that Spine supports skewed scale, bone coordinate systems can be skewed. worldRotation has changed to worldRotationX. There is also a worldRotationY, which is not necessarily perpendicular to the bone's world X axis.
appliedRotation is the local rotation used to actually pose the bones. This can differ from rotation (which is the local rotation) if a constraint (like IK) is applied.
worldSignX/Y are 1 or -1, where -1 indicates the bone has been scaled negatively.