The Parts view is a good place to define the viewing order of an item's part. This can be done in other views, like the structure tree, but renumbering in the structure tree can sometimes be confusing, since the tree view does not separate parts of different part types.
The order of the parts in structure tree is based on the numbering order of the parts, so if you renumber in the Parts view, the order in the tree view will also be changed (and vice versa).
Note also that parts are ordered only within the group of the same part type, which all share a common numbering sequence. In the below example, there are four different part types each with their own sequence.
There are three ways to change the ordering number of parts:
Select the part, and use Alt+Up (key) or Alt+Down shortcuts. The selected part will be renumbered and move up and down.
Note that you can only move parts this way if there is no sorting active.
A fast way to move a part a long way is to do Ctrl+C (copy) on the part, then select another part and use the Move here option. The copied part will then be put above the selected part (and all parts below will be renumbered automatically).
A fast way to renumber many parts alphabetically is to first sort them, for example based on the Part Name or Item Name. Then, click on a part, and use the Renumber option. All parts of the part type will be renumbered (but not the other parts).