“Dependency hell is a colloquial term for the frustration of some software users who have installed software packages which have dependencies on specific versions of other software packages.”
Source : Wikipedia
It’s often tempting to spin up a new trial online instance to do some custom development. The intention is normally to use it temporarily, then export your masterpiece as an unmanaged solution so you can import it into a target vanilla non-trial, more permanent instance.
With Dynamics 365, if you check all the boxes when setting up, CRM will be provisioned with a lot of supplementary solutions as well on top of what you know as a vanilla CRM instance – think Project Service, Field Service and maybe a few extra dependent solutions thrown in there for luck too.