First, let’s clarify some terms.
Kmeta (sulfite) is used to inhibit wild yeasts and bacterial agents from getting a foothold in our wine.Most commercial yeasts have some sulfite tolerance.
Sorbate is birth control for yeast, it prevents yeast from budding and reproducing a colony capable of fermentation. It will not stop an active fermentation, but will prevent a new one from getting started if you add sugar to your wine.
In the winemaking world, exclusive of the kit world, “secondary fermentation” refers to MLF (Malolactic Fermentation), a bacterial fermentation that converts harsh malic acid into smoother lactic acid. Most of us who make red wine from grapes conduct MLF either during alcoholic fermentation, or immediately following AF, by adding the bacteria to cause this to occur. Kmeta inhibits MLF, so we seek to minimize, if not avoid its use until MLF is completed. The presence of sorbate in a wine that undergoes MLF will ruin the wine, period. So the answer where kits are NOT concerned is no Kmeta (or very little) and no sorbate, period.
Hop over to the kit world. In this world, secondary fermentation doesn’t refer to MLF, but instead, to the completion of alcoholic in a SECOND vessel, typically a carboy or demijohn. They call this “secondary fermentation”, even though it refers to the completion of alcoholic fermentation in a second vessel. This terminology discrepancy created by the kit instructions is the source of much confusion. Even in the kit world, no Kmeta or sorbate need be used until AF is complete.
Short and sweet, with kits and grapes and fruits and vegetables, alcoholic fermentation is conducted until the wine is nearly out of sugar, then we move our wine to a new vessel to finish AF, a vessel that helps us prevent too much oxygen exposure as fermentation ceases, and the CO2 that protects it from oxygen is no longer being created. No Kmeta or sorbate needs to be used until your AF is 100% complete. Sorbate is unnecessary if AF has used up all of your sugar and you don’t plan to add sugar to make your wine sweeter.