Basic features, specifying height/width, distances and differences in position.
Specees let you measure the height and width of whole selection of objects. Different stroke alignment can be resolved with visual bound mode. Also, specifying a rectangle's round corner radius is easy.
You can apply scales to dimension in Specees. What's more, you can use variable in scale field to apply any value or unit string in making dimensions. There is also a trick to specify a object height / width in percentage.
Specees can also let you make specification of fonts & color properties in Adobe Illustrator. You can easily make a nice and clean style specification with it.