Crudely classifying these two,
"Length" is a intrinsic function operated of variables
"Length OF" is a special register
Coding either Function Length(variable_name) or LENGTH OF Variable-Name would return the same result? the length of variable-Name in bytes.
Whereas the LENGTH function can only be used where arithmetic expressions are allowed, the LENGTH OF special register can be used in a greater variety of contexts.
For example, the LENGTH OF special register can be used as an argument
to an intrinsic function that allows integer arguments. (An intrinsic function cannot be used as an operand to the LENGTH OF special register.) The LENGTH OF special register can also be used as a parameter in a CALL statement.