Extended modules > Relations > Relation sets > Valid reference elements and focus
  
Valid reference elements and focus
Most relations are defined between reference elements within the assembly or within the component itself. A reference element, for example, can be a face, point, line/edge, cylinder, sphere, or a plane/workplane.
When defining value or logical relations, you are prompted to select the geometry (reference elements) between which the relation will apply. Once you have done this, a menu is displayed containing your selected element types, together with a default focus for each. You can change the foci if you wish.
Valid reference elements are:
points
lines
circles
planes
workplanes
cylinders
spheres
tori
cones
b-splines
spun b-splines
swept b-splines
* 
Swept and Spun B-spline can only be used as reference elements in angle relations, parallel relations, perpendicular relations, fixed relations, and rigid relations.
Within the reference element selected, you can also define a sub-element, or focus. This specifies the reference position more accurately. For instance, if you select a cylinder, you can choose whether the focus should be the cylinder (that is, its surface) or its axis. If you select a circle, you can choose a focus from either its center, or its axis.
Valid options for focus are:
points
lines
circles
planes
cylinders
spheres
axes
directions
centers