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Here, I have selected both Invoice Number fields in each project. You can also use any System Fields if that is desired.
After you have selected your desired criteria, click the Save button at the top left.
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Your search will then be saved and you can make any changes, or save new Global Searches.
To use your Global Search that you have configured, navigate to the Quick Search bar at the top of the screen.
Click on the second icon that looks like a filing cabinet.
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Select the Global Search option.
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You will see now that the text has changed and should now say Global Search.
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The name of the current Global Search filter will be shown to the right of the search bar. You can use the dropdown arrow to switch between multiple Global Searches if you have more than one option in your environment.