Mode A selected on the transponder provides identification of the aircraft to the secondary radar; Mode C will add an automatic height read-out of an aircraft. Mode S is a selective addressing. But previously there was a Mode B which nowadays is not in used anymore. What was it used for, and what information was this mode able to provide?
From a 1984 edition of Doc 9426 (ATS):
For civil aviation purposes, ICAO has designated four, differently spaced, pairs of interrogator pulses known as Modes A, B, C and D [...]
Currently both Modes B and D are reserved for future international use with the stipulation that, if future expansion requires the use of an additional mode, Mode D will be used ahead of Mode B.
SSR modes B and D haven't been used, except mode B in UK, Australia and some other countries for some time. From Radar Theory for Area/Approach Radar Controllers:
In reply to interrogation in Mode-A and Mode-B information of identity, which is set by the pilot, is sent to the ground interrogator.
ICAO Doc 9684 which is the reference for SSR doesn't even mention modes B and D anymore.
Mode B specificity
Mode B used a side-lobe suppression (SLS) feature in order to prevent transponders to reply unless they were receiving the interrogation via the main lobe signal (side lobe signals are undesired by-products of the radiation pattern). This was an improvement over the initial mode A which standard was published 3 years ago, and revised a year ago:
To perform SLS, an additional pulse (P2) was sent by an omnidirectional antenna on the ground, its power was a bit more than the power of the side-lobes, but less than the power in the main lobe.
The transponder did reply only when the interrogation pulses were stronger than this additional pulse. The difference on the display (left: with SLS, right: without SLS):
It was soon replaced by mode C to include altitude telemetry. Mode C had SLS included. For a transponder handling multiple modes, the correct mode must be selected in parallel of the squawk code.
The interrogator (SSR ground station) can request identifiers using one mode at a time, and receive answers from transponders set to that mode. Aircraft using code 1234 will only reply when interrogated using the mode their are currently set to.
Military and civil modes identification pulses
SSR interrogation modes are identified by the timing between pulse P1 and P3 (P2 is the SLS pulse).
SSR modes identification, adapted from source
While mode A is compliant with military mode 3 (both often referred to as 3/A), mode B is purely civil.
SSR has transitioned to mode S for a large part of the centers. Mode S inherits some of the features of the legacy modes, but focuses on data exchange between interrogator and transponder. Pulses play a secondary role, data are exchanged using differentially-coded BPSK. In this system, there is little benefit of using "modes", the content of the dataframe being self-identified.
Additional information about mode B experiments:
Mode B is also used as an antenna test pattern measurement within the FAA MSSR type systems. The MSSR interrogates a dual transponder test set with a mode B, which initiates the test pattern. This is used for troubleshooting and antenna health checks. Mode D is selectable as well, but currently not utilized.