If a full set of branch lengths is not present in the tree file this will be indicated, and you then cannot change this option. If branch lengths are not used, all branches will be given equal lengths of 1.0. Initially a Fixed label angle of 0.0 degrees will be used. If you want to change the way the labels are drawn, the program will offer you a choice between Fixed, Radial, and Along as the ways the angles of the labels are to be determined. However there are additional options.
Radial indicates that the labels are all aligned to as to point toward the root node of the tree.
Along aligns them to have the same angle as the branch connected to that tip. This is particularly likely to keep the labels from colliding, but it may give a misleading impression that the final branch is long. This is initially 90.0 degrees. The angle is read out counterclockwise from the right side of the tree, so that increasing this angle will rotate the tree counterclockwise, and decreasing it will rotate it clockwise. This is by default 360.0 degrees.