FilePosition is a Cardinal property that indicates the current offset for the Data in the packet. 

In the FSP protocol, small packet sizes are used when sending or receiving data in the FSP protocol exchange. As a result, several packets may be required to fully transfer the content for CC_GET_FILE, CC_GET_DIR, CC_PUT_FILE, and other FSP commands. FilePosition indicates the current offset into the complete data stream represented by the Data in the packet. 

FilePosition is used in the WritePacket and ReadPacket methods when property values are converted to / from their binary representations. 

FilePosition is updated during execution of methods that implement the various commands using the logic specified in the FSP protocol.

WritePacket 

ReadPacket