Every pass may bind up to four inputs: iChannel0 through iChannel3.
An input selects:
channel = 0..3
source = another pass, an asset, "keyboard", or "music"
frame = "current" or "previous" (pass inputs only)
filter = "nearest", "linear", or "mipmap"
wrap = "clamp" or "repeat"
Use:
shadertoy inspect channels image --json
to see exactly what a pass consumes.