This means when you use a tweet shortening service (either directly within Twitter or using a tool such as Bit.ly) each unique address e.g., /#01, /#02 will be given a different shortened link. Without this, every time you tweet a link to the same page it generates the same shortened link making it difficult to attribute activity to individual Tweets.
A good example of an organisation that puts this in to the practise is the BBC with the @BBCBreaking account. Depending on how big a story is, there might be several tweets linking to the same article. An example of this was the arrest of Max Clifford.
@BBCBreaking sent out two tweets, one at 1:11pm and one at 1:19pm on Thursday 6th Dec. Both were to the same webpage but as there were dummy details added, they have separate short-codes:
|Click-Through volumes from the two tweets. The first is around 40% higher overall but the response curves have similar patterns|
If you're promoting the same article/page on numerous occasions, this method is a way of tracking the individual piece of activity that has driven that click. This principle works across wherever you put the weblink (Twitter/Facebook/LinkedIn/Email etc.,) and gives you a lower level of detail than would otherwise be available.