Applied to tourist visa for two years and was issued a D visa?

[kimaysa1]

My father applied for a UK tourist visa for 2 years duration, and he received his passport back with the visa type being D and under observations it says “ indefinite leave to enter the UK”. What does this mean? Can he stay fully for the duration of this visa? He applied with my mother, but she received a C visa with observation being “No work or resource to public funds. Duration 180 days”