I've copied this from my reply to a previous post.
1.5 is an unofficial 'catchall' category which covers several different types of diabetes
1)perhaps most often on this forum, it refers to LADA (latent autoimmune diabetes in adults) which is officially type 1. It's a form that has a slower onset in adults than classic type 1 and is quite frequently misdiagnosed in the early stages as type 2 . As it becomes more understood, it is being realised that 10% or more of adults diagnosed with diabetes may have this form.
2) it's sometimes used for MODY (mature onset diabetes of the young) which is a relatively rare genetic form of diabetes. It's neither type 1 nor type 2 (and isn't always diagnosed in the young!)
3) Its sometimes used for double diabetes, people who have type 1 and then become insulin resistant or occasionally children who are diagnosed with both insulin dependency and resistance.
There are probably some other forms that are called 1.5, it really isn't a brilliant term as it can lead to confusion.
Oh and welcome