When sheet metal makes a transition from a bend to a flat surface or to another bend it tends to rip and tear.
Sheet metal corner relief size.
If you use the close 2 bend corner tool you can take the two corner types shown above and they will look like the closed corner shown below with the flat pattern to the right.
For a 3 width ratio when the part is 1 mm in thickness the width of the slot will be 3 mm.
The way this one works is that you select the tool first found on the home tab sheet metal group.
In general a minimum bend relief is equal to the material thickness plus the inside bend radius.
Generally capabilities of of 0 9mm 20mm in thickness are able to be manufactured from sheet 3mm or plate 3mm but this tolerance depends mainly on the part.
This option is straight forward.