What do brackets in an chemical equation mean?
Conventionally for concentrations and activities, square brackets [ ] are used around the chemical molecular formula. For an arbitrary atom, generic letters in upright non-bold typeface such as A, B, R, X or Y etc. are often used.
What do square brackets in an chemical equation mean for molecules?
chemical molecular formula used around the chemical molecular formula
Defining equation (physical chemistry)
Molecules with multiple functional groups that are the same may be expressed by enclosing the repeated group in round brackets. For example, isobutane may be written (CH)CH. This condensed structural formula implies a different connectivity from other molecules that can be formed using the same atoms in the same proportions (isomers). The formula (CH)CH implies a central carbon atom connected to one hydrogen atom and three CH groups. The same number of atoms of each element (10 hydrogens and 4 carbons, or CH) may be used to make a straight chain molecule, "n"-butane: CHCHCHCH.
What do round brackets in an chemical equation mean?
Molecules with multiple functional groups that are the same
Chemical formula
For more complex ions, brackets [ ] are often used to enclose the ionic formula, as in [BH], which is found in compounds such as Cs[BH]. Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in [Co(NH)]Cl. Here, (NH) indicates that the ion contains six NH groups bonded to cobalt, and [ ] encloses the entire formula of the ion with charge +3. 
What do square brackets in an chemical equation mean for ions?
used to enclose the ionic formula
Chemical formula
Defining equation (physical chemistry)
https://en.wikipedia.org/wiki/Defining%20equation%20%28physical%20chemistry%29
Chemical formula
https://en.wikipedia.org/wiki/Chemical%20formula
Bracket
https://en.wikipedia.org/wiki/Bracket
In a chemical equation square brackets are used around the chemical molecular formula. Molecules with multiple functional groups that are the same may be expressed by enclosing the repeated group in round brackets. And for more complex ions, the square brackets are used to enclose the used to enclose the ionic formula.