Key TS Daycount in refers to the convention used during curve building in the following sense:
At each peg date, the quantity defined by the entry is computed so that its implied discount factor matches the one derived by the so far bootstrapped market instruments.
With the sole exception of = Discount, this computation needs the daycount convention provided here, in order to express the involved date-bound time intervals in annual units.
It turns out that this daycount entry has no effect on the produced discount factors for maturities that match the given pegs.
It does have a very small effect on the produced discount factors for maturities that fall between the given pegs, due to a slight change of the interpolated quantity.
An important exception holds in the case where a flat interest rate is used, since no interpolated quantity then exists.
In that case, the daycount entered here is directly used to calculate the discount factor for any given maturity and has accordingly a big effect on the produced curve.