How to understand the sentence below about rapid or fast exchange between vapour and droplet?
The wording is not clear, but here is what I understand:

In case of extremely rapid exchange of molecules between the droplet and the surrounding vapour, the droplet gets the same O18 abundance as the vapour because they reach equilibrium.

In case of extremely slow exchange of molecules between the droplet and the surrounding vapour, the droplet keeps the O18 abundance that the vapour had where the droplet was formed. This is a different location (lower in the front, as shown by figure 2), so the abundance can also be different.