Senior and Subordinate interests in private MBS sold in Tranches were structured into multiple tiers that separate the credit risks into risk priority groupings with higher risk pieces priced to provide higher yields and lower risk pieces sold at lower yields. As in issuer of the private MBS the mortgage lender could retain a piece especially if it was thought to be more valuable than the price available in the secondary market. In that case, the asset would be on the balance sheet, so in this case, any such assets would have been sold to JPM.