Purpose: To check if an SAP screen element is read-only or not.
isprotected - Will return 1 if it is read-only and 0 if it is editable.
Syntax:if(<"F[LIKP-BLDAT]">.isprotected)
// Checks the 'Document Date' field on the Picking screen. If it is read-only then Document already posted set("V[z_vl02n_docdatestatus]","X");
NOTE - Below is the link of another forum article where it is used:
http://www.guixt.com/forum/index.php?topic=81.msg85#msg85