ALMA has a maximum of 8 GHz of bandwidth, this being made up of 4 2-GHz wide basebands. Within each of these, a user is able to place a single spectral window, either a low spectral resolution (TDM; 64-256 channels over 2 GHz) or a high spectral resolution (FDM; up to 8192 channels over a width that ranges from 62.5 MHz to 2 GHz) mode. Actually, the real bandwidths are somewhat smaller than this: the FDM modes in particular are a factor 15/16 smaller than the nominal bandwidth due to the need to overlap the individual 62.5 GHz-wide filters that together produce wider spectral windows. The total usable bandwidth of any mode is limited to 1.875 GHz due to the anti-aliasing filter.
To make things more complicated, the basebands need to be placed inside the sidebands of the particular ALMA band that is being used. Even worse, the location (width and separation) of each sideband differs from band to band, mainly depending on whether they are a two-sideband (2SB) or a double-sideband (DSB) receiver. Most bands, including Cycle-1 bands 3 and 7, have 4-GHz wide sidebands that are separated by 8 GHz. The final kick in the teeth is that, except for the DSB receivers, the basebands must be paired i.e. all must go in one sideband, or two must be in each; a 3/1 split is not possible.
Full details of how this all works can be found in the ALMA Technical Handbook, but it should be clear that spectral setups can be very complicated indeed, especially given that in the future it will be possible to have multiple spectral windows and multiple correlator modes per baseband. The good news is that the OT understands the various restrictions and will not allow a spectral setup to be submitted that doesn't conform to the known capabilities of the telescope during the relevant Cycle. In addition, the Visual Spectral Editor (Chapter 9) is an invaluable tool as it displays the ALMA bands, sidebands and spectral windows. Use it!