It is saying that if you add manganese sulfate, and you don’t want to be declared “out of compliance” when they test your product, then you must meet the strict rule where you product actually contains 100% of the amount you say on the label.
Then it says that if you merely have naturally occurring manganese in the product, and you don’t want to be declared “out of compliance” when they test you, then you merely need to have 80% of the amount you claim on the label… unless you ALSO included manganese sulfate, in which case you got back to the stricter 100% rule.
Which basically agrees with what wezaleff said.
However, I’ll note that this discussion and the quoted rules are only about compliance - it does not actually say what levels should be listed. It just says that if they test and you’re lower than you put on the label, you’re out of compliance.
If you read all of the document and the referenced documents on testing and sampling, they indicate that for a nutrient that is being listed on the label, you should be making the best effort to list the amount actually in the total composite product. I have yet to find a specific line that says this is a requirement, and it seems that under-reporting is acceptable as long as you do not exceed the vague reference to “reasonable excesses.”
So, summing up into plain English:
- If you include an element or vitamin as a supplement, you gotta list that item.
- They say (elsewhere) that you should try to list approximately the total amount actually in the composite product.
- Since it’s a supplement, when they actually test the product, you better have at least 100% much as you list.
- You can have a little more than you list, but only as long as it’s “reasonable.”
Adding these up, you can infer that you need to include the naturally-occurring part when you list it, or else you run the risk that when they test your product, and the amount comes in way higher than the amount you list they may say your excess is “not reasonable.”
Frustratingly, this means manufacturers must also be careful about which assays the FDA uses when testing… because if the given assay will detect one form and not another, you need to know that to determine how much to put on your label! For (hypothetical) example, if the given assay would detect both manganese sulfate and the naturally-occurring organically bound manganese, the label should show the total of both. But if they’re using an assay which would only detect the manganese sulfate and would not detect the organically-bound stuff, then you should only include the amount from the manganese sulfate on the label - or else, when they test, they’ll find you out of compliance!