title: .EACHI in data.table?
body: <p>I cannot seem to find any documentation on what exactly <code>.EACHI</code> does in <code>data.table</code>. I see a brief mention of it in the documentation:</p>
  
  <blockquote>
    <p>Aggregation for a subset of known groups is particularly efficient
    when passing those groups in i and setting <code>by=.EACHI</code>. When <code>i</code> is a
    data.table, <code>DT[i,j,by=.EACHI]</code> evaluates <code>j</code> for the groups of <code>DT</code> that
    each row in <code>i</code> joins to. We call this grouping by each i.</p>
  </blockquote>
  
  <p>But what does "groups" in the context of <code>DT</code> mean? Is a group determined by the key that is set on <code>DT</code>? Is the group every distinct row that uses all the columns as the key? I fully understand how to run something like <code>DT[i,j,by=my_grouping_variable]</code> but am confused as to how <code>.EACHI</code> would work. Could someone explain please?</p>