This functionality was modified in an update. For more information, see Filter column headings in a matrix (modified in an update).
You can use the Filter option to restrict the number of displayed rows in the matrix. You can filter by multiple columns.
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In the Workflow Management feature set, select Document matrices.
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Select the matrix and select Actions > Assign Recipients.
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Select
next to the column header you want to filter, and select Filter 
to see the filtering options.
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Type the criteria.
Select And to combine conditions. Select Or to specify that one condition be met.
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The applied column filters will be cleared when you add or duplicate a row.
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The currently selected row will be cleared when you apply a filter.
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When you close the Assign Recipients window, the filters are not preserved.
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With the filter turned on, if you edit the property values and save the changes, the system reloads the matrix rows based on the filter criteria.
Show and hide column headings
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Select
next to the column header, and select Columns 
to see the column headings available for display.
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Select a column heading to show it in the matrix.
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Clear the column heading to hide it in the matrix.
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When you add or remove a column, all the hidden columns will be visible.
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You can undo the current changes by selecting RESET. However, you cannot undo the changes after selecting APPLY.
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When you close the Assign Recipients window, the hidden columns are not preserved.