From the README:

Similar to std.MultiArrayList() but backed by fixed size arrays with a shared runtime length.

Each field of T becomes an array with max length buffer_capacity. To access the arrays as slices use items(field) or constItems(field).

Useful when a struct of small arrays is desired with capacity that is known at compile time. Like std.BoundedArray, MultiBoundedArrays are only values and thus may be copied.