question about bcp documentation

I’m trying to dynamically generate a non-xml format file with bcp utility.

The command I’ve been give is:

bcp SwiftUsage format nul -c -f "$ tempdir"/table.fmt -t,

What’s nul? And where is this documented?

I ask because the format file that it generates is consistent with the column ordering on the sql table side, but is inconsistent with that data I’m trying to import. I’ve figured out how to manually edit the format file to achieve the correct ordering, but I’m trying to automate this, so that’s not acceptable. So far, I’ve found no documentation for the argument. It’s just given as a constant.