Reference Plane or Reference Line?
Reference Planes and Lines are used to constrain the line elements when creating a 3d object or 2d/3d lines themselves or to guide critical dimensions in the project. It can be found in the Work Plane panel which is in Architecture tap in the project or in the Datum panel in Create tap in the family editor.
Reference Plane is a 2-dimensional surface (like a plan view) to draw 2d lines. It is important to note that it is an independent work plane from the original which means you can set it up wherever you want as long as you have a reference. It is used as a guideline when creating a family or in a project. There are a few hierarchies in the reference planes that will determine which reference planes will get snapped first in the project.
Strong Reference - High priority for dimensioning and snapping
Weak reference - Second priority for dimensioning and snapping
Not A Reference - no dimensioning and snapping
The intersection of the reference planes with 'define Origin' will be an insertion/origin point in the project.
The 2d icon on each end of the plane will allow you to adjust the length in the view specifically, whereas the 3d icon will adjust it in all views visible. If you want to change back to the previous, you can reset it to 3d extent.
Reference Line is a guideline which has 4 reference planes whereas curved lines offer 2 reference planes. It is acting like 2D or 3D lines but is not visible in the project when is loaded. It is normally used to constraint angles (rotation) in the family editor if needed (I prefer to just use 2d/3d line and constraint to the reference plane).
Name it properly, for instance, Front, Back, Left, Right, Top, Bottom, etc.
Assign appropriate 'Subcategory' and 'Is Reference' parameters.
Carefully pin/unpin the reference plane. It will impact how the family behave parametrically.
As the reference planes are a bit easier to use to constrain the elements, I primarily use the reference planes for the family creation and Reference Line is not available in the project! Just note that do not abuse using the reference planes in the project as it is extremely difficult to manage them.