(The full requirements are described in the ISO/IEC Directives Part 2:2011, 6.6.10.)

• The symbols used in each formula should be defined, either in the text above or immediately below the formula.
• Symbols for variable quantities are to be written in italics.
  
  Note: We recommend the use of Times New Roman type so as to avoid confusion between the letters (uppercase i) and (lowercase L), for example.
• Subscripts are written in Arial upright type, unless they represent symbols for variable quantities (in which case, they are written in italics).
• If it is necessary to number some or all of the formulae in a document in order to facilitate cross-reference, Arabic numbers in parentheses are used, beginning with (1).
• Symbols, rather than descriptive names, shall be used in formulae.

Related topics:

• Writing equations and formulae
• How to apply the subscript / superscript styles from the IEC template