The quarterly distributions are classified as part interest, part capital gains or losses (both long and short term), part return of capital, part allocated profit, and sometimes other things. You will know your allocation when you get your K-1. Just enter the numbers into TurboTax or import them directly. Easy to do.

If you hold for a long time and your capital account basis goes to zero then all distributions are taxable 100%.

If you sell your entire position your capital account basis is your taxable basis for paying taxes. Some ordinary income and some capital gains. If you only sell part of your units any deferred capital loss continues to be deferred. You also cannot elect which units to sell. The MLP generally uses an average basis for allocating cost.