Argument explicitness in `inner`

Stream: mathlib4

[Jovan Gerbscheid]

Are there documented guidelines on when arguments should be explicit and when implicit? I'm a bit confused about the choice of explicit variables in these:

inner : E → E → 𝕜
innerₛₗ (𝕜) : E →ₗ⋆[𝕜] E →ₗ[𝕜] 𝕜
innerₗ (F) : F →ₗ[ℝ] F →ₗ[ℝ] ℝ

[Johan Commelin]

It depends on whether you think this is a bundled object that will often be passed around in unapplied fashion.
E.g. if you want to use the bilinear form API, then you want something like innerₗ F, so that the F is determined without having to apply the form to some element of F.