Interest income would be shown on Line 1 of the Schedule K-1, so we know it is not interest income. 

If it is on Line 5 of the Schedule K-1, we know it could be any one of a number of things.  What I usually see there are proceeds from cashing in an annuity or IRA.  We know it's not rental income, because that would be on Line 7 of the Schedule K-1.

Some things on Line 5 are subject to the extra 3.8% tax on rich people.  But Box 14 is telling you that it's not subject to the additional tax, so you can subtract it out on the Form 8960.  But you don't have to worry about that unless your client is rich.  Your client still has to pay income tax on the Line 5 amount, just not the 3.8% surtax.  

Letters that come with Schedules K-1 should be ignored.  They might be propaganda planted by North Korean operatives.  IRS does not look at letters, it only looks at the Schedule K-1.  If you think the Schedule K-1 is wrong, write the 1041 preparer a letter.