Some fields, e.g. Invoice No or Userdef6 are text fields and you can write in whatever you want so long as you have access. Other fields, especially at row level, are table fields where you can search or and enter data which is in a table - e.g. Supplier or Project. Enter part of the code or the description to get a list of suggestions. So if you want account 6360 - Cleaning in Debit, you can write in "63" in the field to see all accounts conaining "63". If you know the whole code you can type it all in. When you have the right value, press ENTER or TAB to select it and go on to the next field.