The bold 'A' and 'T' stand for accessible toilet, with imagine of a toilet for those who aren’t unfamiliar with the logo. This helps those from other countries to understand the room is a toilet.
Braille is used to assist people with visual impairments, also the bold green is to help make the logo visible from a distance.
The base of the logo is a 3D 'A T', this is to assist people with communication and sensory/learning difficulties, who can be assisted by a carer to feel the 'A T' and understand they are being supported into an accessible toilet.