Under the current Geometric Dimensioning and Tolerance System in use in North America today, (ANSI 14.5 1982?) I've never seen the -C- used as it is within the dialogue box in the upper right. It presumably references datum -C-, but should only be used as it is shown in the dialogue box in the lower right area of the print (without the - - on either side of the C). The current system has no symbols or convention that I am aware of to denote "symmetry".
Note that the features within a box are known as "basic" dimensions, which have no tolerance of their own. They are known as "gage" dimensions from which other dimensions or features are derived.
Note that the features within a box are known as "basic" dimensions, which have no tolerance of their own. They are known as "gage" dimensions from which other dimensions or features are derived.