This article is from the ZTerm FAQ, by Les Jones firstname.lastname@example.org with numerous contributions by others.
What some BBSes call YModem is actually XModem-1K. If the BBS
offers YModem and YModem Batch, use YModem Batch, which is the true
YModem ZTerm expects. Otherwise, if YModem transfers aren't
working, tell the BBS to send YModem, and tell ZTerm to receive
XModem-1K. Frankly, this is just one more reason to use ZModem.