A Sequence is logically composed of three things: an array of elements, a maximum number of elements that the array may contain (i.e. its allocated size), and a logical length indicating how many of the allocated elements are valid. The length may vary dynamically between 0 and the maximum (inclusive); it is not permissible to access an element at an index greater than or equal to the length.
A Sequence may either “own” the memory associated with it, or it may “borrow” that memory. If a Sequence owns its own memory, then the sequence itself will allocate the its memory and is permitted to grow and shrink that memory (i.e. change its maximum) dynamically.
Source: Sequence