[Agda] Difference between parameterized and indexed datatypes

[v0id_NULL]

Dear list,

Help me please. I can not understand what difference between these
definitions.

data T1 {ℓ} : Set ℓ where
 c1 : T1

data T2 : {ℓ : _} → Set ℓ where
 c2 : T2

Why first one is approved but second one is rejected with error
"The sort of T2 cannot depend on its indices in the type {ℓ :
Agda.Primitive.Level} → Set ℓ when checking the definition of T2"?
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