 Allocation to labelled bins

Allocate $n$ distinct objects to $k$ labelled bins of specified nonnegative sizes $n_1,\dots ,n_k$, with ${\sum }_i{n}_i=n$. Order within a bin is ignored. The number of allocations is the multinomial coefficientTo prove this, order all the objects in a permutation and assign the first $n_1$ to the first bin, the next $n_2$ to the second, and so on. Every allocation comes from exactly $n_1!\cdots n_k!$ internal orderings. These disjoint classes exhaust the $n!$ permutations, proving the formula by the multiplication principle. Empty bins are allowed because $0!=1$.

For two bins of sizes $m$ and $n-m$, the count is $\left(\genfrac{}{}{0ex}{}{n}{m}\right)$ and every allocation has multiplicity $m!(n-m)!$ in the permutation sample space. Exchanging two equal-sized labelled bins generally gives a different allocation. In contrast, a set partition uses unlabelled nonempty blocks; forgetting bin labels changes the counting problem and must not be done silently.