FindPosition[A1,A2]

A1, A2: names of defined observers

The function FindPosition returns the position vector from the reference point of the observer A1 to the reference point of the observer A2.

Restart;
DeclareObservers[A,B];
DeclarePoints[A,B];
DeclareTriads[a,b];
DefineObservers[{A,A,a},{B,B,b}];
DefinePoints[A,B,a,1,2,3];
FindPosition[A,B]

$\left\{''Type''\to ''Vector'',''Size''\to 1,1\to \left\{''Coordinates''\to \left\{1,2,3\right\},''Triad''\to a\right\}\right\}$

DeclareTriads, DeclareObservers, DeclarePoints, DefineObservers, DefinePoints, FindTranslation, FindCoordinates