When you load a file version into a ClearCase® view, the ClearCase Remote Client applies the local file system's read-only attribute to the file. When you check out the file, the read-only attribute is removed so that you can make modifications. When you check in the file, the read-only attribute is reapplied.
If you need to modify a loaded file without checking it out (for example, if you cannot connect to a ClearCase server), you can use the hijack operation to make the file modifiable even though it cannot be checked out.