NIRCam's two modules—A and B—provide redundancy and double the available field of view in the imaging and wide field slitless spectroscopy (WFSS) observing modes. For observing modes covering smaller fields of view, a single module, A or B, may be used, based on optimal performance. The two modules have nearly identical optical elements and detectors with a few notable differences.
Notable differences between modules
Module B has better throughput at the longest wavelengths (see comparisons at NIRCam filters page). This is most pronounced for the F480M filter, which has ~33% and ~42% throughput in modules A and B, respectively (this includes all telescope throughputs, filter transmissions, and detector efficiencies).
Module A grisms have anti-reflection coating on both sides. Module B grisms are only coated on their flat (non-grooved) sides, reducing their efficiency by ~26% and introducing some ghosts of bright spectra (Greene et al., 2016).
The module A coronagraph optical substrate has a significantly better anti-reflection coating than the module B substrate. The module A coronagraph mask neutral density squares are also superior. Coronagraph mask occulters are arranged in different orders in each module (both sets are equally good).
Field of view
In both modules, the fields of view of the short wavelength (SW) and long wavelengh (LW) channels are almost, but not entirely, identical. However, module A provides slightly better overlap. The slight difference in overlap will not affect most observations, but may impact observations that use the smallest subarrays.
Views of the NIRCam modules
Greene, T., Chu, L., Hodapp, K. W., et al. 2016, SPIE 99040E
Slitless Spectroscopy with the James Webb Space Telescope Near-Infrared Camera (JWST NIRCam)
Greenhouse, M. et al. 2011, SPIE 814606G
Status of the James Webb Space Telescope integrated science instrument module system