What is Reference Length in Y-Plus Calculator?
 

I have seen in all of the y-plus calculators there is an input for length named as either 'reference length' (or) 'boundary layer length' (or) just 'length'.

eg. for which are said to be "pipe diameter or vehicle's length", which was written in one of the websites for y-plus calculator.

From what I understand,
for an internal flow simulation it is the diameter of the fluid's flow path (assuming it to be cylindrical) and
for an external flow simulation it is the overall length of the object or geometry for which the flow is being simulated on (eg. chord length for an airfoil, vehicle's length).

Is my understanding correct?