A dimensional constraint applied within the Block Editor using the BCPARAMETER command is called a constraint parameter.
While you can use both dimensional constraints and constraint parameters in a block definition, only constraint parameters will display editable custom properties for that block reference.
A horizontal constraint parameter controls the X distance between two points on one object, or between two objects.
A vertical constraint controls the Y distance between two points on one object, or between two objects.
An aligned constraint controls the distance between two points on an object, one point and one object, or two line segments.
An angular constraint controls the angle between two line or polyline segments, or the angle of an arc.
A radial constraint parameter controls the radius of a circle, arc, or polyline arc segment.
A diameter constraint parameter controls the radius of a circle, arc, or polyline arc segment.