To add to The-Lu's great visual answer with some extra technical detail for SVG format.
If you use that example SVG picture: http://www.plantuml.com/plantuml/svg/VT91oeGW5C3nlQSGJw1Ft4N4zvr4J6AeT7ZPQfYxZsdI39YkYl_l2JqQjg0nxEj2jldXdXhrIl-qcIVqknkDNppIWDfjCLaNo1T3Qu9sqsBfYz1uqiI_HsUnwz3mlgV7uRK8PuHQ57bKr2TpX6gKESfgL3cgQXJyg89N8wG8ARu9kJwSnKVsytuDKixYJUScxWmK0_T6S9QDQ5rK43IGB4Wsa2f8DP3WKPMztIee2f8Qh1lZd_O1
Then a modern browser (I tried Edge and Opera) will let you right-click and 'inspect element'. You can then highlight the different 'rect' components in the text sidebar and it will highlight the rectangle it refers to in the graphic. You'll see that the rects with rounded corners have parameters 'rx' and 'ry' that are both set to half the roundCorner value. rx and ry define the radii of an ellipse at the corner - and since both are set the same you get a circle (until you max one out). The value is an absolute amount (as opposed to being relative to the shape size) and I think the default unit for svg is pixels. The maximum meaningful size for rx is half the width of the rectangle at which point you get no straight line part. Similarly for ry and the height.
Take the "roundCorner 50" example:
rect[Attributes Style] {
- fill: rgb(254, 254, 206);
- filter: url(#fi4dkuyxad8cj);
- height: 52.5938;
- rx: 25;
- ry: 25;
- width: 132;
- x: 7;
- y: 233;
}
rx and ry are set to half of 50. Since that is almost exactly half the height , you get no perceivable straight line at the sides.
