If you have an object built only of triangles (e.g., imported from another program), Cinema 4D can try to convert the triangles into quadrangles. This works only as long as the triangles that you want to convert will result in a planar quadrangle. Triangles that cannot be converted are left in their original state. So, Untriangulate works only for triangles that result in planar quadrangles.
Untriangulate does not affect n-gons.
This option affects the creation of quads and defines the angle at which the resulting quad will be created between two triangles.
Activate this option if you want to use the triangles to create a common N-gon. The value defined in the Angle setting will be used automatically.
Use this setting to define the angle the algorithm should use to determine at which point two triangles should be combined to create a common N-gon. Defining very high values here will almost always lead to extremely non-planar polygons (in the case of N-gons it will result in simplified geometry with fewer points), which is something you want to avoid.