Can the /s sub-layer of the stereochemical layer appear in more than one layer?
In general, /s may appear in M, MI, F, and FI. However, you may want to take a look at the “Appendix 2. Abbreviations and Layer Precedence”, section “a. Layer Precedence” (InChI Tech. Man.) The reason is that /s segment (as well as /t, /b, and /m) may be deliberately omitted if it is same as in the preceding layer. For stereochemical layers the precedence is somewhat complicated because it depends on the existence (that is, non-emptiness) of the layers in M or F. One more (not quite obvious) reason for omitting a contribution from a non-isotopic component to an isotopic stereo segment /t or /b (located MI or FI) is that this component, unlike isotopic components, *always* has isotopic stereo exactly same as its non-isotopic stereo in the preceding layer. The contribution from such a non-isotopic component to the isotopic layer /m is “.”, as if the inversion does not change its /t stereo. /s may not be omitted in, for example, MI if some of the components do not have /t in M and have /t
