The dialog sidebar is located on the right side of a boardlet and is optically a layer that opens from the bottom of the boardlet. It can be closed with an "x" icon in the upper right corner, just like the user is used to in other software.
Every dialog sidebar has a sidebar title that describes the context of the dialog.

For styles see https://app.zeplin.io/project/5e9dacd70b95bc52259f2f94/styleguide/components?seid=5ec2e220e16994494ce3acca

go to section ID3 Dialog