It is sometimes desirable to convert member offsets from local axes to global axes or vice-versa. For example, if some members have offsets in global axes and you rotate the model then the globally offset members may finish up offset in the wrong direction relative to the rest of the model. In such cases you could convert them to local axes before performing the rotation. You could also convert them back to global axes after the rotation if you wanted to.
In order to change the axes of members with offsets you should select the members, right-click and then select "Member Tools" => "Convert Offsets to Global" or "Convert Offsets to Local" from the popup menu that appears.
Any members that have no offsets or which are already in the desired offset axes will be left untouched during the conversion process.