You have 2 buckets of money in TSP.
The first is what you currently have in your account. If you want to redistribute the money that you already have in your account between different TSP funds, you execute an Inter-Fund Transfer (IFT).
The second bucket of money is the new money that you contribute each payday. To tell the TSP which funds to buy with this new money, you do a Change of Fund Allocation.
We recommend doing both a change of fund allocation AND an IFT at the same time. That way, all of your TSP dollars are in/going to the funds that you want. It makes it very simple to keep track.
If your IFT is 100% S fund then 100% of money in your account gets distributed to the S fund. If your fund allocation is 100% G fund then, a month later, when you look at your account page at TSP.gov, you’ll wonder why you have money in the G fund when you did an IFT to the S fund. It’s because you told TSP to put your new contributions into the G fund.