Thank you Froggiest.
I suspected this, as I asked for in "Reply #4457".
I also searched for details, but didn't find the rule.
But, IMHO, it seems something is missing: if retargeting asks necessarily for an active sensor, and having "Terminal Guidance" is not enough to trigger retargeting (i.e., it isn't an AS), when we select "Retarget Capability" in the missile design, I think it should automatically also require that "Active Sensor" must be <> 0. So, at least a warning message should appear, or eventually we shouldn't be allowed to "Create" the design, if AS is 0.
Thermal or EM sensors would they work too, instead of AS?