This is how I have explained it to faculty in the past.

Re-medial.  The term means that you have done it, did not do it well and we will be applying a treatment (AGAIN) to it to get it right.

Developmental means you are growing and this is new to you and you will learn it and internalize it for your future use.  Consider your target population.  If they are new freshmen perhaps most of what you do is new to them, especially if it involves study skills and transitions.  If it is just subject, content material you may have to assess to determine if you are fixing something or introducing something. 

I always remind facualty that when they started their terminal degree (Ph.D./Ed.D), they chose a group of persons to help them through the process.  That was developmental.