FX is a kind of Foreign Exchange transaction where a currency is exchanged into other currency. In Japan there are three factors of profit and loss.
Contents
1. Fluctuation in price (profit or loss）
Exchange rate fluctuation. For example, when we exchange YEN into USD, we win if USD price is up, or we loose if USD price is down. The profit or loss will be fixed when we exchange USD back into YEN.
2. Swap Points (profit or loss)
Equivalent to interest and earned every trading day. As policy interest rate is different among countries, we earn profit when low interest rate currency is exchanged into high one, and we suffer loss when high rate currency is exchange into low. For example, if we exchange YEN into USD, we will have earned profit until USD is exchanged back into YEN as of January, 2020. Swap Points differ among FX providers and change day by day.
3. Transaction Fee (only loss)
In Japan, most of FX providers charge only “spread”, a kind of transaction fee, to us but some of them do another fee on top of it. “Spread” is difference between prices of bid and ask as an image below. (Bid Price in exchanging USD into YEN, Ask Price in exchanging YEN into USD) It’s charged in new transaction but not done in settlement one. 
We can earn profit by “1. fluctuation in price” and/or “2. swap points”. Generally, the fluctuation affects on our profit or loss more than swap points.
