**Edited:** to reflect the correct definitions. 

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**Question 1:** Why are the guts well-defined?

**Answer 1:** By the JSJ theory there is a unique collection of $I$-bundles (and Seifert fibered spaces) that contain (up to isotopy) all essential product disks and product annuli (and all essential tori).  We throw away the $I$-bundles. Note that any Seifert fibered pieces remaining are pared solid tori.  (There are no interesting Seifert fibered pieces because the original manifold $M$ has "non-degenerate" Thurston norm.  So all essential tori in $M$ are parallel to boundary components.)

**Question 2:** Why is the pared guts well defined?

**Answer 2:** For exactly the same reason.

**Question 3:** What is the relation between pared guts and the sutured guts?

**Answer 3:** Pared guts are directed at understanding geometry.  Sutured guts have an additional homological condition (the assumption of tautness).

**Question 4:** Why do we need to consider product disks in the sutured case, but not in the pared case?

**Answer 4:** We don't need them in either case.  Consider the frontier  of a regular neighbourhood of the union of a product disk and the sutures it meets.