The Dimensions window contains the list of all dimensions and their hierarchies within the cube for the currently selected measure group.
By default, the hierarchies are grouped by Dimension and Display Folder (this is configurable), however, the Measures dimension is always listed at the top.
You can expand or collapse a single node or the full list by clicking Expand/Collapse or Expand All/Collapse All from the context menu for a node or from the options menu.
Beneath every hierarchy, the current selection for the hierarchy is displayed. Every hierarchy has a default selection, which is what is initially displayed for the hierarchy. If you explicitly set the selection for a hierarchy, the name of the hierarchy will be displayed in bold. This makes is easier to visually identify hierarchies that have an explicit selection.
If the selection is too large to fit in the area beneath the hierarchy, hovering above the hierarchy will display a tip will the full selection listed in it as displayed above.
Note: it is also possible to list all hierarchies with an explicit selection on top, see options.
Opening the Dimensions window
By default, when creating a new report, the Dimensions window will automatically be opened. If you close it, by clicking on the cross in the top-right corner, you can reopen the window again by clicking on the 
button in the ribbon.
Opening the selection window for a hierarchy
To open the hierarchy editor for a hierarchy, click on the name of the hierarchy. When hovering above a hierarchy, the name will be underlined and the cursor will change to a hand pointer. You can also open the editor by right clicking on a hierarchy name and selecting Modify Selection...
To reset the selection for all hierarchies, also those included on the axes, click the 
button on the toolbar.
Adding a hierarchy to an axis
A hierarchy can be added to an axis by clicking on the hierarchy name and dragging it to the grid. You can also right-click on the hierarchy name and select the corresponding Add to <axis> command.
See adding a hierarchy to an axis for more information.
The following options are available when working with the dimensions window:
Show Autofilter 
When this option is on, an extra text entry box is displayed at the top of the list. Typing any characters in this area will filter the list to include only those hierarchies that include the characters typed. This makes it easy to find hierarchies when dealing with a cube with many hierarchies.
When the results are filtered, a status bar will be displayed with a filtered message. The status bar also contains a Clear button that allows you to remove the filtering with a single click.
Show Dimensions
When set, the hierarchies will be grouped in the list by the dimension they belong to.
Show Display Folders 
When set, the hierarchies will be grouped in the list by the display folder they belong to.
Show Selection
When set, the current hierarchy selection will be displayed beneath the hierarchy name in the list.
A hierarchy has an implicit selection when an explicit selection has not been made, but the default member for the hierarchy is not an (All) member. When this option is set, hierarchies that have an implicit selection will be displayed in a bold gray color. Also, when Show Active on Top is selected, hierarchies with implicit selections will be displayed between hierarchies with an explicit selection and hierarchies with no selection.
Show Active on Top 
When set, all hierarchies with an active (i.e. non-default) selection are displayed at the top of the list. If the selection for a hierarchy is changed from the default, it will automatically be moved to the top of the list. This makes it easier to view all background hierarchies with an active selection at once, without having to scroll the list. If Highlight Implicit Selection is turned on, hierarchies with an implicit selection will also be moved to the top of the list, but always under those hierarchies with an explicit selection.