Each ISI journal has one or more Editors-in-Chief (EiC) and Associate EiCs. The EiC conducts the initial review of each submission to gauge if the paper might have the potential for publication after revision. Only submissions that pass this initial quality control step move on for an external review. For papers that pass this initial assessment, the EiC assigns an Editor to chair the ad hoc paper review panel and reviewers for this paper. Your Assignment Letter is sent to you by the EiC. 

There is also a Managing Editor who works with issues in using the review system, not editorial issues.

Understand that the EiC considers many factors in assigning reviewers to papers. The EiC may have selected you as a specialist or as a non-specialist (since the paper’s Editor needs to hear both voices). That is, the system compares what you have selected as your topics of interest and expertise and the topics the author selects to describe the submission. Another factor is your availability.  The EiC looks to see if you are currently reviewing other papers and the system keeps track of dates that you showed in your profile that you are unable to accept review assignments.  For more on this, see the section  “OOPS! I WAS ASSIGNED A PAPER WHEN I AM UNAVAILABLE.”