Welcome to the concept of edit locking, unstored calculations and fields with global storage.
If you click into a field on a given record, Filemaker "locks" that record against edits by other users. If they attempt to modify a field in the database, they get an error message telling them that they cannot edit the record until you commit the record to release it.
Global fields and certain unstored calculations, however display values unique to each user. If you modify a value in a global field, only you will see that change and the value in the global field reverts back to it's original value when you close the file. The only way to make a permanent change to value in a global field is to make the change on the host computer.
An unstored calculation field that uses a get function that checks for settings, such as the account name that are specific to a given client will act just like a global field, each user will see the value in this field consistent with the settings on their computer. Get ( LayoutName ) or get ( ActiveFieldName ) to name just two examples evaluate in the same manner.
Learn and use this principle: Test your idea and accept the result. Never assume something works the way you think it should.
For instance, you could have 5 people open the same record, find record id 12, and try to change a field's value. FileMaker will show you how it works.
You will also discover that these 5 people can pull data from the same record simultaneously such as the cost of a product for new records.