Fb = M/S = Mc/I (conceptual edition)

gents, i seem to be having a wording issue. or a conceptual issue. or both. maybe it is because i am used to using the terms “allowable” or “required” or “actual” with these equations?! what is S please? it is in unit of in^3. what is the “practice oriented usage” of S please? i am GIVEN a required Fb in #\in^2 based upon some given live load and dead load? yes? what is M? it is an ALLOWABLE PSI for the extreme fiber of some arbitrary beam? yes? so now i have to GIVENS and i solve for the SECRION MODULUS S? and i look this up in the steel or wood or whatever chart and find a “SECTION MODULUS” that will support this LOAD with this MATERIAL? yes? what are the “conceptual implications” of this? that if i somehow double the ACTUAL LOAD on any particular floor in the building i am therefore increasing M by 2 so i will then have to find structural members with DOUBLE the value of S to well “solve the equation”? or to “support the doubled floor load” since the earlier one will no longer support the load presumably? i mean i seem to feel like this is an if/then question when it should be a “actual/required” wording for me to understand it. can anyone untangle that for me? i can’t seem to replicate the syntax anywhere in these books...