How Sakpata etc over cap on MDT with no MDT on them and the DT values not reaching 50 (DT cap).
DT = MDT, PDT and BDT
Shell Provides MDT specifically 11%/16%/22%/26%/29% (sheltered ring adds shell tier/256 value to the original value/256. 11%/17%/23%/27%/31%)
The 50 DT Cap encompasses all forms of BDT, MDT, PDT.
(PDT + DT can never surpass 50 total PDT Value.*
MDT + DT can never surpass 50 total MDT Value.*
BDT + DT can never surpass 50 total BDT Value.)**
With ShellV providing 29% or 31% MDT, Dring providing 10 MDT, You need to find at most 11% MDT or DT to cap.
Whereas what Eiryl was saying is you need to find multiple pieces in order to cap PDT which is why Ambu capes (by and large) favor PDT Augments.
I hope this answered your questions about DT that you may have had.
*= The only exceptions to these rules are jobs with items that provide stats that are in the II category, IE: MDT II and PDT II.
**=Pet DT Values have caps that are not limited by the 50% cap players have and are subject to different modifiers and formulae. IE: Pet: BDT/DT/MDT/PDT and Pet Specific damage modifiers/reductions.