use_data_table() imports the data.table() function from the data.table package, as well as several important symbols: :=, .SD, .BY, .N, .I, .GRP, .NGRP, .EACHI. This is a minimal setup and you can learn much more in the "Importing data.table" vignette: https://rdatatable.gitlab.io/data.table/articles/datatable-importing.html. In addition to importing these functions, use_data_table() also blocks the usage of data.table in the Depends field of the DESCRIPTION file; data.table should be used as an imported or suggested package only. See this discussion.
use_data_table()