Fixing Days

Key Fixing Days in
Term Rate refers to an integer that affects the start date of the spanning time interval that defines the term rate, as described at Term Rate

As mentioned in the above link, the "spanning time interval" [T₁,T₂] may differ from the "accrual time interval" [Tᵃ₁,Tᵃ₂] over which the term rate accrues the interest amount that is eventually paid out.
By default, the rate r is fixed business days before the special reference date - known as value date - Tⱽ, where nˢ ≥ 0 is the rate's intrinsic settlement delay in business days wrt that rate's calendar.
Tⱽ usually equals the start Tᵃ₁ or the end Tᵃ₂ of the corresponding accrual interval [Tᵃ₁,Tᵃ₂]
So, the rate r is usually fixed at time T₀ with T₀ = Tⱽ - nˢ

The pattern described above applies if the entry here is left blank.
A non-blank integer entry here overrides this pattern by forcing the reset date to fall on any custom date and not necessarily business days before Tⱽ

Concretely, assume a - perhaps even negative - integer n is defined here.
Then the reset date T₀ changes so that T₀ = Tⱽ - n, in the sense that n calendar days are subtracted from Tⱽ
This change then affects the spanning interval [T₁,T₂] as the start time T₁ is always constructed as T₁ = T₀ + nˢ
If the tenor in
Tenor is defined, the T₂ is adjusted so that the length of [T₁,T₂] stays the same and not affected by the entry here.
But if the tenor is not defined, it is likely that the length of the spanning interval [T₁,T₂] also changes.
Note, the entry
Settlement still plays a role since it defines the

Also note that the accrual period [Tᵃ₁,Tᵃ₂] is not affected by the entry here.