SOC 1 stemmed from the original SAS 70 report, which, once SSAE 16 was issued in April 2010, the formal report name was changed to being a SOC 1 report (but issued under the SSAE 16 guidance) and effective as of June 2011. SSAE 18 was then issued May 2017 and will be effective as of December 2018, and apply to all SOC 1 reports issued thereafter.
Previously in SSAE-16 a Management Representation Letter was highly recommended and common practice, however, they were not explicitly required to be obtained except in certain existing subject matter sections. This is now consistent across all sections.
SSAE-18 will be effective for all reports issued after May 1, 2017.