Re: Purpose of generate-id index [1]

/TDLfile[generate-id(.)=generate-id(
        key('TimingByGroup',concat(@type,'::',@period))[1])]

In general terms why is the [1] index present.

As you noticed, it is redundant. Two reasons why it is still
commonly used:
1. Make it explicit to human readers that the first element
  of the node set returned by key() is used. Not everybody
  has always all details of the spec at hand.
2. Moderatly clever processors might take the index as a clue
  not to construct the whole node set, which can save time and
  memory. Really clever processors could take the same clue
  from the presence of the generate-id() function of course,
  but the former is still a bit easier to implement and also
  more worth to do because the optimization is also an improvement
  in many more situations.