Caution : Do not assume these measurements are correct at this stage, they need additional testing and proofing, make sure you have verified that the software calculates the measurements correctly yourself before using them for navigation.
Great Circle Distance
Gives the shortest distance between 2 points on the earths surface. The heading when travelling between them is constantly changing.
Rhumb Line Distance
Rhumb lines or loxodromes give the distance between 2 points on the earths surface when travelling between them at a constant heading. This is not the shortest distance between the points.
For small distances the 2 methods give the same values.