For REIT+ and Cash+ Flexi portfolio, your funds are invested in SGD-denominated assets. On the other hand, in Core, Select, and Cash+ Flexi USD portfolios, the assets are typically denominated in USD.
If your investment is denominated in a different currency from the funds you have transferred, exchange rate fluctuations may affect your returns - this is called the 'Currency impact'. Cash+ Flexi USD portfolio value and transactions are always displayed in USD, and you can check portfolio transactions for the FX rates that your SGD deposits were converted at.
You can learn more about currency movements and the effect on your portfolio returns here.