When you make a character formatting change (eg bold, italic) or a paragraph formatting change (eg bullets or line spacing) Word inserts a vertical line in the outer margin (similar to when a text change is made).
However when you make a character formatting change such as font, font size, alignment then no vertical line is displayed.
A border applied to characters however does insert a vertical line
By default character formatting changes are not indicated with a vertical line ?
The reason for this is to avoid the confusion with whether a change was a content change or a formatting change.
You can specify a change mark and color for when Word makes formatting changes to help distinguish between the two.
Change the "Changed Formatting/Mark" box
Formatting  Color 
Example / Sample
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