The function Flatten allows you to explicitly flatten out all sublists.
FlattenAt lets you specify at what positions you want sublists flattened.
Sequence objects automatically get spliced in, and do not require any explicit flattening.
|Sequence[e1,e2,…]||a sequence of arguments that will automatically be spliced into any function|
Sequence works in any function.
Here is a common way that Sequence is used.