Tiptop Z-DSP - Manual (Page 8)

Digital Feedback.
I N S E R T C A R T R I D G E
Some algorithms process feedback internally, in the
digital domain. The texture of digital feedback is much
different and brings a very different flavor than analog
feedback. Combining analog and digital feedback will
bring even more depth to a sound.
You’ll know that a program is using digital feedback
from FDBK showing on the display as a parameter.