When files are uploaded or downloaded, it is possible for them to be corrupted, and various mechanisms for checking that this has not occurred are built into FileCOPA.
The client and server each create a checksum of the file in question, and ensure that these are the same.
Checksums are most simply understood as adding up every bit in the file, using complex algorithms that should avoid the same checksum being created for more than one file.
This feature is used by FTP clients that do file synchronisation - checking whether a changed file on the client has been backed up on the server. The client creates a checksum for a file, commands the server to do the same for this file, and if the two are different, the file needs to be copied over. If not, then the two versions of the file are identical.
FileCOPA supports XMD5, XSHA1 and XCRC - these are transparent to you as the server administrator, and are accessed silently by the FTP client.
When a connection is first set up, the client can issue a FEAT command which asks for features the server supports. Afterwards, the client will use any it supports that are appropriate to the context.