Optional fields in ORC file objects

[Richard Evans]

Hello

Reading the message specifications in the ORC file format 
(https://orc.apache.org/specification/ORCv1/) I see every field is 
marked optional.  Yet some of the fields are always required (such as 
the footerLength in the file Postscript).

So are all fields always present?  Or is there any way to tell which 
fields are truly optional and which are always required?

Thanks

Richard