You may have seen many file types with different extensions such as STEP versus STP, IGES versus IGS.
In the past, due to changing operating systems, there were restrictions on filenames. So there was different set of file extensions.
There was a time when UNIX and VMS were the main operating system’s in the business. There was no limit to the length of file extensions.
For DOS file names and files on windows platform, there was restriction to the 8.3 filename standard (aka SFN or Short File Name). This standard allowed maximum three characters after a period (.) as a part of file name extension. However, this was existed only prior to Windows 95 and Windows NT 3.51.
Some companies had chosen the full four character (or more) file extensions on their VMS or UNIX operating system which was accepted standard at that time. With windows adoption the applications that were available on only VMS or UNIX, the file extension was truncated to the limit of three characters.
With Windows 95 and Windows NT 3.51, the file extensions were not limited to three characters any longer. Today we see both the three-character filenames and the filenames with extensions greater than three characters.
So technically STEP, STP and IGES, IGS are the same with different file extensions. All major CAD software’s like Siemens NX, Dassault Systèmes SOLIDWORKS, Siemens Solid Edge, PTC Creo, Autodesk Inventor, Dassault Systèmes Catia list/support such kind of STEP (STP), IGES (IGS) files with different extensions.