Use 'tell off' for mild to moderate scolding, not for severe punishment.

Severity

'Tell off' is generally used in informal situations, often among friends or family.

Informal

Often used when someone in authority scolds a person under their charge.

Use 'tell off' to describe organizing items or people into distinct groups.

It's used to clarify that separation is for the purpose of organization or distribution.

Implies that the division is done fairly or equally among groups or sections.