NetCDF-4 files can store attributes, variables, and dimensions in hierarchical groups.

This allows the user to create a structure much like a Unix file system. In netCDF, each group gets an ncid. Opening or creating a file returns the ncid for the root group (which is named “/”). Groups can be added with the nc_def_grp() function. Get the number of groups, and their ncids, with the nc_inq_grps() function.

Dimensions are scoped such that they are visible to all child groups. For example, you can define a dimension in the root group, and use its dimension id when defining a variable in a sub-group.

Attributes defined as NC_GLOBAL apply to the group, not the entire file.

The degenerate case, in which only the root group is used, corresponds exactly with the classic data mode, before groups were introduced.