My understanding is never add sulfite while any type of fermentation is active because it hampers the fermenation.
I've seen people ask about it when transferring from primary to secondary. The confusion may come because some people leave everything in the primary until fermentation is complete and stable below 0.998. In this case racking into a carboy is not for secondary fermentation, and only for degassing and clearing, so K-meta (sulfite) is added, along with Sorbate to stabilize the wine. This is common with Skeeter Pee and Dragon Blood because they are fast fermenters and early drinkers. For grape wine, once all fermentation is complete, and the wine is in a carboy to bulk age, then it needs a little sulfite every so often, and racking is a good time to add it, especially since the racking operation itself will introduce some oxygen.
That's my understanding only... from reading, not from doing.
I've seen people ask about it when transferring from primary to secondary. The confusion may come because some people leave everything in the primary until fermentation is complete and stable below 0.998. In this case racking into a carboy is not for secondary fermentation, and only for degassing and clearing, so K-meta (sulfite) is added, along with Sorbate to stabilize the wine. This is common with Skeeter Pee and Dragon Blood because they are fast fermenters and early drinkers. For grape wine, once all fermentation is complete, and the wine is in a carboy to bulk age, then it needs a little sulfite every so often, and racking is a good time to add it, especially since the racking operation itself will introduce some oxygen.
That's my understanding only... from reading, not from doing.