Almost precisely
.
This (handy) extension was added in FreeBSD 5.0-release afaik, most other dump implementations that support an -L have a different meaning to it, or lack the flag all together. For example, certein flavours use -L to label the dumps with a short string. For any real measure of portability with dump, one would likely have to constrain to the UNIX version 6 or version 7 manuals list of arguments; which doesn't even support the hyphens convention; e.g. something like dump 0uf dump0-usr /dev/ad0s1f rather then dump -0u -f dump0-usr /dev/ad0s1f.
Since dump isn't standardized even that much is dangerous without reading the local systems manual page first, and hoping it is accurate. Which more or less makes most 'fun' invocations of dump in scripts, inheritly implementation defined behaviour.
and likely worth a disclaimer, depending on how you feel about supporting occasional (stupid) cookie cutter admins, who will likely whine if it didn't work on box xxx or nuked their hard drive 8=).
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Thou shalt check the array bounds of all strings (indeed, all arrays), for surely where thou typest ``foo'' someone someday shall type ``supercalifragilisticexpialidocious''.