What Is Assertion?

To assert something is to perform a certain kind of act. This act is different in kind both from other speech acts, like questions, requests, commands, promises, and apologies, and from acts that are not speech acts, like toast buttering and inarticulate yodeling. My question, then is this: what features of an act qualify it as an assertion, and not one of these other kinds of act? To focus on a particular example: in uttering “Bill will close the window,” one might be practicing English pronunciation, asserting that Bill will close the window, or requesting that Bill close the window. What makes it the case that one is doing one of these and not another?