The dotted circle, in Unicode ◌ DOTTED CIRCLE (HTML
◌), is a typographic character used to illustrate the effect of a combining mark, such as a diacritic mark. In Windows, it is possible to use the key combination + to produce the character.
A Unicode combining mark combines with a preceding character. When used as stand-alone, it would combine unintentionally with a preceding character (possibly a space):
Using the generic dotted circle character also shows the relative positioning of the diacritic.
Combining characters are shown with a dotted circle. [...] the relative position of the dotted circle indicates an [...] approximate location of the base character in relation to the combining mark. [...]