In the context of file compression, a "piece" or "part" is a chunk of a larger file that has been split into smaller sizes for easier uploading and downloading.
The filename refers to a specific segment of a split archive for the game Serious Sam 4 . Breakdown of the Filename Serious_Sam_4_v1.09-Razor1911.part09.rar
If you only have part09 , you cannot run the game or see its contents; it is just one fragment of the complete data set. In the context of file compression, a "piece"