What does "Rxlch" mean in ENCODE?

In ENCODE project, we could see some files are called "...rxlch...", which means "reverse crosslinked Chromatin".

What does this mean?

According to the protocal of Myers' lab in Hudson Alpha Institute, It is used as the control for the TF ChIP-seq.

Ref:

[1]https://bhe9.wordpress.com/2014/01/17/chip-control-experiment-for-hudsonalpha-lab-haib-on-encode/

[2]http://myers.hudsonalpha.org/documents/Myers%20Lab%20ChIP-seq%20Protocol%20v041610.pdf