Creating stems or sub-mixes involves recording virtual instruments in hardware so initial signal processing can be applied. Audio is routed within the audio interface, which provides a more acceptable result than routing via analogue cables.
Drums are recorded in mono, with ambience in stereo, while basses and guitars remain mono. Keyboards are usually in stereo with the exception of leads. Orchestral sounds are in stereo to support stereo imaging. Stems are truncated for fades and minimised for latency.
Stems may be a combination of interleaved stereo or mono files, however recordings are mixed and rendered in stereo, with 24-bit resolution, for better than average sound quality, which is mastered and returns as a 16-bit audio file, with a 44.1KHz sample rate.
Sub mixes or stems of various parts, for example, Strings, Reed, Brass and Mallets are created for simplicity, since sounds in each category can be mixed together during the recording process, rather than creating additional files for mixing.