For concreteness, let's assume that you are using 437 as your OEM code page (which as we all know is not actually provided by the OEM) and 1252 as your ANSI code page (which as we all known is not actually the product of the American National Standards Institute).
You can use Alt+0154 to type a Latin small letter s with caron because position 154 in code page 1252 is the Latin small letter s with caron. On the other hand, lowercase r with caron does not exist in code page 1252, nor does it exist in code page 437, so if you want to type that character, you're out of luck. The Alt+nnn sequence lets you type characters from the OEM code page, and Alt+0nnn lets you type characters from the ANSI code page, but if the character you want is in neither of them, then those sequences aren't going to help you.
(As an experiment, I didn't write any motivating discussion. It's actually easier for me because coming up with a narrative to accompany a dry technical article is hard work. If I don't have to do it, so much the better for me.)
[Raymond is currently away; this message was pre-recorded.]