explanation.

Tangent Circle to 2 circles and Radius between line and Arc require additional information which is provided by checking radio box for each circle located on the top of form

Choosing between external and internal depending on position tangent circle (or radius in case of radius between 2 lines function) to entities.

Red Circle represent tangent (which we are looking for) circle. Like You may see function not require information which one of the circles is inside or outside other just needs relative position.

It means You should check internal if one circle is inside other but You don't have to specify which one is Inside.

Same rules apply to Radius between Arc and Line Function.