Write up / write down - this is currency valueation. You have deposit in USD, but your local currency is CHF. So you need to prepare your reports in your local currency - CHM.
At the end of month you need to do valueation (tr. TPM1).
At the begging of you deposit (13.03) you have 30 000 000 USD ( 29096843.03 CHM). Exchange rate is 1.0310396893941.
On 31.03 exchange rate changed: it became lower than exchange rate on 13.03. So the system made write down.
On 30.04 exchange reate changed: it became higher than exchange reate on 13.03. So the system made write up.
The same till happen on 31.05. But we don't know exchange rate on this date. So we don't know whether it will be write up or down.