When you compress a file or folder with WinRAR, you have the option to split it into smaller parts. This is one of those parts, and requires all of the other parts to extract the archive. Other valid WinRAR Archive Partial File extensions include:
.R01, .R02, .R03, .R04, .R05, .R06, .R07, .R08, .R09, .R10, .R11, .R12, .R13, .R14, .R15, .R16, .R17, .R18, .R19, .R20, .R21, .R22, .R23, .R24, .R25, .R26, .R27, .R28 and .R29
