A Stub IOS is an IOS that serves no purpose other than a placeholder. They are released to overwrite other IOSes, either to overwrite a cIOS in a specific slot and break homebrew, or to remove an unused IOS, freeing up space and removing the need to maintain the unused branch as new exploits are found.
Stub IOSes generally have 3 contents: a 0x40 byte build tag, and two shared contents, which depend on the variant. The first shared content generally contains ES, while the second contains FFSP and the kernel (including IOSP).
There are several variants of stub IOSes.
The first stub IOSes were released in 3.3rev03 to IOS4, IOS10, IOS11 and IOS20, with a size of 245760. They were built by someone named
yasuh-to@TONKAT. This variant was also used for the IOS30 stub distributed in 3.4, and the IOS3 stub that was never released on NUS.
The 2008 variant was based on IOS51, although the IOS51 stub was not of this variant.
IOS70 has size 245760, but has nothing in common with the 2008 variant of stub IOSes. It was built by
saito_tomoya@IR and is based on the functional version of IOS70.