Normally, when a sentence ends in a capital letter, the space following the period is treated like a normal space. To get an end-of-sentence space, you must put \@ before the period.
Is this also required if the capital letter is enclosed in a command, such as \emph{}, \texttt{}, or \textbf{}?
For example:
The pointer cannot be \texttt{NULL}. There is one exception:
vs.
The pointer cannot be \texttt{NULL}\@. There is one exception:
An image is worth many words.
The assignment of the space factor code is global, so whether or not you change fonts or use groups, after the last L the space factor code would be 999, meaning that the period would be interpreted as following an uppercase letter.
I set a very large \xspaceskip in order to emphasize the effect produced by the space factor after a period, which is 3000 if it was 1000 or more before it.
So \@ is necessary.
The pointer cannot be \texttt{NULL}.\the\spacefactor There is one exception:, where you can verify that the spacefactor is 1000 and not 3000 as you'd want after a period.) Of course it is easy to construct a command where it isn't needed (\newcommand*{\foo}[1]{#1\@}or\newcommand*{\goo}[1]{#1 goo}). Generally I'd expect the space factor to prevail unless the macro typesets additional text or explicitly changes the space factor.()or[]. E.g., compare the outputs ofbe (NULL). There isvs.be (NULL)\@. There is, or the outputs ofbe [NULL]. There isvs.be [NULL]\@. There is.