The TIFF file format uses 32bit offsets and, as such, is limited to 4 gigabytes. This has been quite sufficient for many years. Today however, there is a need for a good multi-purpose open image file format that can handle huge images, or very large collections of images, breaking the 4 gig boundary.
There is currently an ongoing attempt to design a new variant of TIFF, called BigTIFF, that closely resembles TIFF, but uses 64bit offsets instead. The benefits of closely resembling TIFF are huge. For instance, existing TIFF libraries can quite easily extend their support for TIFF to also include this new variant. Documentation needs are minimal. All the much appreciated properties of a file format that has been around and has been extended for more then a decade are inherited. All properly known tags are being reused, all supported bitdepths and datatypes remain valid. The arbitrary number of 'extra channels', the tiling and striping schemes, the multitude of compression schemes, and the private tag scheme, that made TIFF very useful in pre-press as well as for storing scientific data, and many other applications, all remain intact.
Yet, the offset bitdepth changes, and BigTIFF files are no longer restrained by the 4 gigabyte limitation from which classic TIFF suffers. More info on http://www.awaresystems.be/imaging/tiff/bigtiff.html