We thought it was in degrees too. But after rotating it to 57º, using AnglePlanes Plugin and rotating to the same angle using Reformat, the results aren’t similar.
57º rotation using AnglePlanes Plugin
57º rotation using reformat (this angle is 32º not 57º)
The same DICOMs were used to create the previous plots, they are just not shown.
I confirm that it is rotation in degrees. Note that rotations are incremental: if you rotate around multiple axes, the previous rotation slider is always reset to 0.
These angles may refer to the same rotation - they are just different angles of the same triangle: 57deg = 90deg - 32deg (-1deg probably measurement error).