The Full Form of ISLB is‍ Indian Standard Light Weight Beams.

They have the same depths but the ISLBs have thinner flanges and webs that make them lighter. But they also have less area and less Moment of Inertia than ISMBs of the same depth and hence have less load and moment carrying capacity.

The differences are merely of academic interest to number crunchers who love to ‘optimise’ their member selection using computers and software and attempt a ‘minimum weight’ design using the properties of ISLBs listed in Tables of Steel Sections.

These exercises are futile because only ISMBs are rolled and available in India.

ISLBs are not rolled in India and hence not available.

There is no point in considering them in one’s calculations for member selection.

Simply forget about ISLBs and stick to ISMBs.

Incidently even ISHBs, ISJBs are not rolled in India.