The front index (Table of Contents) of the 271 transaction has the LS (Loop Header) under Loop ID - 2115 Subscriber Eligibility or Benefit Additional Information. However the actual segment on page 328 (2115C) and page 431 (2115D) has the example listed as LS*2120.
We believe that the TOC is accurate and your example incorrectly states 2120 in place of 2115.
This can cause major problems when Payers implement the transaction and different 5010 validation softwares interpreting it differently.
This issue is explicitly addressed in final guide 005010X279, dated April 2008. The LS segment sits between the 2115 and 2120 loops. It is part of the 2110 loop. When used, it marks the beginning of the usage of the 2120 loop which sits between the LS and the related LE (which is also part of the 2110 loop). The front matter, examples and segment detail are all correct.
See section B.1.1.3.11.1 in the guide for a description of bounded loops.