Well, I'm not sure about the second part, but for the first part I think Artha is asking why does Sigil use <b></b> when selecting a subset of words in a paragraph/division and hitting the bold button, but uses an sgc class in the header when selecting the whole paragraph/division to mark bold. Why doesn't it always use the class approach?
(On an unrelated note, this just reminded me that bold/italic/underline confuse me on occassion
- have you considered making the labels B / I / U ?)