Documents represent the data you have added to an ATLAS.ti project. These can be text, image, audio, video or geographic materials that you wish to analyze. Working with text data includes importing survey data, importing data from a reference manager for a literature review, importing twitter data, using interview or focus group data, reports in text or PDF formats, observational notes, etc. For more information, see Supported file formats.
Document groups fulfill a special function as they can be regarded as quasi dichotomous variables. You can group all female interviewees into a document group named "gender::female," all male interviewees into a group named "gender::male." You can do the same for different professions, marital status, education levels, etc. See Working with Groups for further information
Document groups can later in the analysis be used to restrict code-based searches like: "Show me all data segments coded with 'attitude towards the environment' but only for females who live in London as compared to females who live in the country side."
You can also use document groups as a filter, for example to reduce other types of output, like a frequency count for codes across a particular group of documents.
This is explained in more detail in the section Code Document Table.