What is IMP bit of Section Descriptor for IXP1200?

In Section 7.6.5 of the ARM Architecture Reference Manual (the ARM ARM), bit 4 of the First-level section descriptor is described as "implementation defined." Does anyone know what this means for the IXP1200 and the IXP1200 evaluation board firmware? The IXP1200 evaluation board firmware sets the IMP bit to 1 for all mappings. Thanks, Scott