title: What does .SD stand for in data.table in R
body: <p><code>.SD</code> looks useful but I do not really know what I am doing with it. What does it stand for? Why is there a preceding period (full stop). What is happening when I use it?</p>
  
  <p>I read: 
  <em><code>.SD</code> is a <code>data.table</code> containing the subset of <code>x</code>'s data for each group, excluding the group column(s). It can be used when grouping by <code>i</code>, when grouping by <code>by</code>, keyed <code>by</code>, and _ad hoc_ <code>by</code></em></p>
  
  <p>Does that mean that the daughter <code>data.table</code>s is held in memory for the next operation?</p>