From AMule Project FAQ
Online Signature is a mono-directional way aMule has to communicate with external utilities (such as CAS, wxCAS, AMPS, etc...).
It consists of a file aMule writes on disc from-time-to-time (by default, every 5 seconds, but this can be changed in "Preferences" -> "Online Signature") to inform about it's current status.
aMule currently supports two signature files formats:
More information about this can be found at aMule's FAQ.