How come there is no error when I use choiceLength without specifying lengthUnits?

[Roger L Costello]

Hi Folks,

I have a choice element in which I specify dfdl:choiceLengthKind="explicit" and 
dfdl:choiceLength="3" but I do not specify dfdl:lengthUnits. I run my schema 
and get no error. Why? I believe that I should get an error. What value is 
there to telling the parser that each branch of the choice is 3?    3 what?    
3 bytes?    3 bits?    3 characters?

/Roger