Self-plagiarism is attempting to pass off your own words as previously unpublished original work when they are not. If you are copying text from a work that is not intended for publication, then it cannot be a prior publication, and thus it cannot be self-plagiarism. Likewise, it is OK to take text from one of your rejected works if you have decided not to publish it. If the other work is not published but is also aiming for publication, however, then it would be self-plagiarism (though which of the two works would be "original" and which "self-plagiarized" could depend on the details of the progress of each toward publication).
Many pieces of text, however, do not expect the work to be original, in which case you can plagiarize, but not self-plagiarize. Taking your example: if you create job application letters by modifying a base template, the duplication from letter to letter is perfectly ethical, because there is no expectation of originality in a job application, just for truthfulness.
Funding applications are a funny in-between state. They do not have the same expectation of originality in text as a publication, but most funding agencies require that you not be attempting to fund the same proposed research through any other application. Thus, you would be safe to use the same preliminary work text as part of pitching two different ideas (though it will probably need to be customized for context in any case). Pitching closely related ideas, however, even with different text, would typically be a violation of the funding agency rules. Since there is a decent chance that some of the same people will see both proposals, it's a fast track to rejection and burning bridges with funders. Don't do it.