What are we referring to when we say 'simple organisms' or 'complex organisms'?

The terms ‘Simple organisms’ and ‘complex organisms’ are used to classify the organisms based on the period of their evolution.

‘Simple Organisms’ are primitive organisms whereas ‘complex organisms’ are advanced organisms.

‘Simple Organisms’ have simple structural and functional organisation whereas ‘complex organisms’ have complex structural and functional organisations.

‘Complex organisms’ evolved from ‘Simple Organisms’.

These terms are used to refer to the complexity of the division of labour.

These terms are also used to refer to the relative level of organisation in the organisms.