Official Description
6.5.3.2 For an indexed-variable closest-containing a single index-expression, it is an error if the value of the index-expression is not assignment-compatible with the index-type of the array-type.
Simplified Description
In other words, it is error to access an array component that does not exist. For example given:
var a : array[1..20] of char;then
a[0]is an error since the index-expression (i.e. 0) is not assignment-compatible with the index-type (i.e. 1..10) of the array type. NOTE: There is no 0th component of the array a.
Error Handling
This error is reported if range checking is enabled.