Manipulating tag-axis
Tags are for attaching other entities to a model. The tag has a position and a direction.
The position is not so hard to figure out for anyone, it is the point where the 3 axis intersect. The direction of a tag is given by the angles of the 3 axis.
The 3 axis, named X- Y- & Z-axis, are respectively coloured Red, Green & Blue. In other words, the red line points in the direction of the (positive) X-axis, the green line points into a direction along the (positive) Y-axis and the blue line points indicates the (positive) Z-axis.
If You rotate the direction of a tag in a model, any attached entity will also be rotated the same way.
With this tool You can invert one tag-axis, or swap 2 tag-axis.
I've loaded the ET-model of a truck: blitzbody.md3, which has 4 tags in the model. The image below shows the back of the truck where the 2 objectives are attached:
 
 
Inverting an axis of a tag
By selecting the choice from the menu, You can invert one of the three axis.
When You choose to invert the X-axis, the red line will flip direction.
 
The result of the invert:
 
 
Swapping tag-axis
It is also possible to swap 2 tag-axis. Just select the proper choice from the menu.
If You swap the X- & Z-axis, the red & blue lines will flip.
 
The result of the swapping of 2 tag-axis: