CCS/TMS570LC4357: what means the bank width of bank of flash

Part Number: TMS570LC4357

Tool/software: Code Composer Studio

hello sir

when I call the API Fapi_issueProgrammingCommand() to write data into the flash, 

like this

Fapi_issueProgrammingCommand((uint32_t*)(gSector_List[num].low_addr + offset), buff, len, 0, 0, Fapi_AutoEccGeneration);

when the len is larger than 8, it will return a false "Fapi_Error_AsyncIncorrectDataBufferLength" ,

so what it is about "bank width"?