A fill pointer is a non-negative integer no larger than the total number of elements in a vector. Not all vectors have fill pointers. See the functions make-array and adjust-array.
An element of a vector is said to be active if it has an index that is greater than or equal to zero, but less than the fill pointer (if any). For an array that has no fill pointer, all elements are considered active.
Only vectors may have fill pointers; multidimensional arrays may not. A multidimensional array that is displaced to a vector that has a fill pointer can be created.