What is the "sequence" and its details as referenced for the segwit application? On the segwit page there is a reference that the "feffffff" is the "sequence".  What does the "sequence" represent in relation to what?  Does it always have to be "feffffff"?  What happens if it's "feffffff" instead of "ffffffff"?  Does it help anything, does it hurt anything?  Am I just fine using just "ffffffff" instead of "feffffff" What other options are there and what do they mean?
Sequence is part of any input, before segwit as well. feffffff is the hexadecimal number 0xfffffffe because the bytes need to be reversed. This sequence number, as per BIP 125, is because:


  Wallets that don't want to signal replaceability should use either a max sequence number (0xffffffff) or a sequence number of (0xffffffff-1) when then also want to use locktime;


So this sequence number uses the locktime but does not signal replaceability

If instead you used 0xffffffff, the sequence number would have no effect on the transaction.

If you don't know what locktime is, check out this question: What is Bitcoin&#39;s locktime feature exactly? How can it be used?