Cite the User Guide handbooks as follows:
[Module name] handbook, GeoDict 2021 from Math2Market GmbH, Germany, https://doi.org/10.30423/userguide.geodict2021-[module name]
(for example: GrainGeo handbook, GeoDict 2021 from Math2Market GmbH, Germany, https://doi.org/10.30423/userguide.geodict2021-graingeo)