RFC parameters (pass by value)

I believe Iam missing something in the concept of passing by value and reference in a RFC FM.

the question/s is/are:

we always have to check <b>'pass value'</b> for parameters(we get a syntax unless), but what value will be passed to the input parameters of the rfc,are they initial values based on type?

and do we still have to give reference when it doesn't make sense in a network?(or once the call is made from an outside system )

RFC:

<b>parameter/ ref / pass value</b>

v_vbeln /like /vbak-vbeln / check

hope i make sense.

i would greatly appreciate any answers.

thanks