In the Dimension Style Advanced Overrides dialog,there are a couple of bugs in Rhino5. (Latest Demo as of today.)
1. There are two check boxes in front of the text 'Use Alternate Text'. The second doesn't seem to do anything.
2. When I check the Number format box, I can select the 'Fractional' and 'Feet & Inches' radio buttons, but they do not stay selected when I click on OK.
3. 'Show Mask' also has two check boxes.
Repro: Select all items in a model with some linear dimensions. In properties select the Dimensions toolbar button, and select Advanced button in the 'Dimension Style Overrides' dialog. Primary text 'Length factor' checked and set to 1.0, First checkbox in front of 'Use Alternate Text' selected, 'Number format' checkbox selected, Alternate text 'Length Factor' checked with an arbitrary value placed in the edit box, and 'Prefix' and 'Suffix' checkboxes selected.
Also, why should anything have to be selected to get to the 'Dimension Style Overrides' dialog? You didn't need to do that with Rhino4. (Everything works as expected in Rhino4).