This margin code is used to denote extensions beyond the ISO C standard For interfaces that are duplicated between IEEE Std 1003 1-2001 and the ISO C standard, a CX introduction block describes the nature of the duplication, with any extensions appropriately CX marked and shaded Where an interface is added to an ISO C standard header, within the header the interface has an appropriate margin marker and shading (for example, CX, XSI, TSF, and so on) and the same marking appears on the reference page in the SYNOPSIS section This enables a programmer to easily identify that the interface is extending an ISO C standard header