“Multi-mode” could be more accurately described as “software definable”. RF and/or IR imagery can be used in different ways depending on the target.

A single software definable RF seeker can have the following modes:
1. Active radar homing
2. Semi-active radar homing
3. Home on jam
4. Anti-radiation homing

A software definable IR seeker can have the following modes:
1. IR imaging
2. IR laser spot tracking

The seeker software can also alter pursuit modes based on target distance (trajectory shaping vs direct flight to target), pursuit profiles depending upon engagement angle (proportaional navigation, direct/tail chase, inertial GPS), fuzing options (ballistic missile [very fast] vs aircraft [relatively slow]) and blind targeting options (e.g. LOAL).

Depending upon the sophistication of microprocessors and memory, the seeker may have different levels of automatic target recognition. And seeker sophistication allows many forms of ECCM.

The sophistication of a software definable seeker is why comparison between the performance of AIM-120 A/B with AIM-120 C/D isn’t valid. Apples and oranges.