Variables, and the values they represent, may change over time. If you have used variables in prior year releases, check the ACS product changes webpage for source table changes. If the variable you have been using comes from a table that has changed from the prior year, check the variables list (variables.html) and compare the variable concept and label to be sure you are requesting the correct estimate.
In September 2016, ACS released annotation variables that return character representations of each estimate. Many annotations return as null. However, if an annotation variable returns a value, it provides important information about the estimate or margin of error. For example, if an estimate variable (variable ending in “E”) returns “-888888888”, the annotation variable will return “(X)”. Looking at the Notes on ACS Estimates and Annotation Values, this means the estimate is not applicable or not available. For a complete list of return values and their annotations, see Notes on ACS Estimates and Annotation Values.