For spectral lines, different choices will depend on the desired velocity resolution. If the source is weaker than 50-100 mJy at centimeter wavelengths, phase referencing must be used and also should be checked. For single polarization observations, check IEEE RCP for 3.6 cm or 13 cm, and IEEE LCP for all other bands. For a multi-band observation including bands that default to LCP as well as bands that default to RCP, both should be checked. Entries with ratios other than 1 generally are needed only for observations with non-VLBA telescopes.
Novices are welcome to ask for extensive help in order to make sure that they get the details right.
The processor is typically the VLBA correlator, and other items are usually blank for single-polarization continuum observations.