The difference between
XBTEUR.d is that the former is on the "regular" kraken exchange whereas the latter is on a dark pool.
In substance, dark pool orders are only visible to the exchange (and the exchange will fill your order as and when somebody comes with an opposite order) but not to other market participants. In theory it allows you to place larger orders without the market knowing about them.
Note: in a regulated environment (say equities), the exchange is not allowed to use the information provided by your order to "front-run" you. I don't think there are any similar laws applicable to crypto-currencies - to be checked.
You can find additional information in the official announcements here and here.