atOffset: and atOffset:put: raise the error OffsetOutOfRange if the offset is less than zero or greater than or equal to the count of members in the collection.
Offsets provide an alternate means to access the members of a collection, without creating a separate index object. Some collection subtypes (such as Array) support fast, direct access by integer member offset, while others support member offsets only as a shorthand for sequential access through every preceding member. Access by offsets is supported on all collections regardless of whether its speed on a particular collection type.
An offset is an integer value that gives relative position of a member in the enumeration sequence of a collection. Offsets start the count of the first member at zero, just like C array indexing.