1.5 isn't an 'official' diagnosis.
Probably most often on here it is used for LADA (Latent autoimmune diabetes in Adults)
It is a slower developing form of type 1 which presents at an older age than 'classic' type 1. For some time (sometimes a short time but sometimes several years ) it isn't insulin dependent but the beta cells are slowly being attacked by the immune system eventually resulting in true insulin dependence. Quite frequently the person is initially diagnosed as having Type 2. Tests for insulin production and for antibodies can help to diagnose it. It's thought that 10% of adults diagnosed with Type 2 actually have LADA.
Sometimes 1.5 is used for MODY (though officially its called 'other' or type 3 because it comes in the third list on the WHO definitions). Maturity Onset Diabetes of the Young is an inherited form of diabetes and at the moment they've found (Ithink) 6 different genetic strains all with slightly different characteristics.
A couple of times I've seen 1.5 used for type 1s who develop insulin resistance but more often this is called 'double diabetes'
Actually it gets more confusing as they discover more types, theres MIDDY, ketosis prone type 2 and not forgetting type 1b (which isn't autoimmune and is on the increase).... I'm sure that there are lots more :!:
I wouldn't be surprised if they decide to redefine the classifications again soon to try to clarify things.