Stocks are cash products, so if you buy a USD-denominated stock (for example from a CHF-denominated account), your profit or loss will also be influenced by how the USDCHF exchange rate changes while you have the position open.
Example:
Let's examine the example of a position in the United States Oil Fund, LP (USO:arcx) where 46 shares were bought on a CHF account on the 11th of December 2014. By the 16th of February, the ETF had fallen from 22.69 to 19.62 – meaning the client was losing on the trade itself.
Besides that, a further loss has been accrued due to the CHF strengthening against the USD over the two months since the shares had been bought, so now the rate applied on the conversion back into the account was less favourable (0.92661365 vs. 0.97438770 when the position was originally opened). These rates take into account any conversion cost applied.