What does a comma signify in inorganic chemistry?

A short “obligatory” reference from IUPAC Recommendations [1, p. 21], section IR-2.2.3.1 regarding use of parentheses in formulae:

(d) In solid-state chemistry, to enclose symbols of atoms occupying the same type of site in a random fashion. The symbols themselves are separated by a comma, with no space.

Example:

9. K(Br,Cl)

Related question on chemistry typesetting in $\mathrm\LaTeX$: Comma in chemical formula.

References

1. IUPAC “Red Book” Nomenclature of Inorganic Chemistry, 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; IUPAC Recommendations; Royal Society of Chemistry: Cambridge, UK, 2005. ISBN 978-0-85404-438-2. PDF

I have seen this nomenclature used as a shorthand to indicate a solid solution of two different ionic compounds that share a common anion. For example:

1. Wang, Q.; Grau-Crespo, R.; de Leeuw, N. H. Mixing Thermodynamics of the Calcite-Structured (Mn,Ca)CO3Solid Solution: A Computer Simulation Study. J. Phys. Chem. B 2011, 115 (47), 13854–13861. DOI: 10.1021/jp200378q.
2. González-López, J.; Ruiz-Hernández, S. E.; Fernández-González, Á.; Jiménez, A.; de Leeuw, N. H.; Grau-Crespo, R. Cobalt incorporation in calcite: Thermochemistry of (Ca,Co)CO 3 solid solutions from density functional theory simulations. Geochim. Cosmochim. Acta 2014, 142, 205–216. DOI: 10.1016/j.gca.2014.07.026.

Since it's a solution of two different compounds, its composition can be variable.

Quoting [1]:

Comma. In accord with general usage, the comma "," should be used to indicate continuous substitution, partial as well as complete, if there is no change of CN. Vacancies (symbol $\square$) are treated here just like atoms. This usage is restricted to chemical formulas for atoms on a specific site. The comma is not appropriate for structure site formulas because different symbols indicate a change in the coordination number. Examples for proper use of the comma include forsterite-fayalite $\ce{[(Mg{,}Fe)2(SiO4)]}$ and hornblende $\ce{\{(Na{,}\square)Ca2(Mg{,}Fe{,}Al)_5[(Si{,}Al)4O11]2(OH)2\}}$. In structure type formulas, the comma is used to mean "or" where substitution may occur. An example is in the zeolites where H2O and M units may both occur.

Comma representation are often used in writing chemical compositions of minerals as the elements and its concentration vary to a certain extent. See minerals.net:

A number of minerals contain a varying amount of two or more elements like Aurichalcite, which has a chemical formula of $\ce{(Zn{,}Cu)5(CO3)2(OH)6}$ contains an unspecific varying amount of zinc (Zn) and copper (Cu). This is indicated by comma separating the Zn from the Cu. If a chemical formula with two elements in parenthesis is separated by a comma, the number of those elements vary. Aurichalcite has a variable amount of zinc of copper where the combination of both these elements totals five. The more dominant element is usually listed first. (Emphasis added.)

For more information, see this previous Chemistry.SE question and see here to find steps regarding how to write such type of formulae using chemical markups.

References:

1. Smith, D. K.; Roberts, A. C.; Bayliss, P.; Liebau, F. A systematic approach to general and structure-type formulas for minerals and other inorganic phases. Am. Mineral. 1998, 83 (1-2), 126–132. DOI: 10.2138/am-1998-1-212. Online version: http://database.iem.ac.ru/mincryst/fst.html