A 7alt chord has neither a natural fifth nor a natural ninth.

It does have a dominant seventh, but the alterations (hence the name, alt) to the fifth and ninth give it an entirely different sound than a straight seventh chord. Examples could include any or all of b5, #5, b9 and/or #9.

You see 7alt all the time in jazz fakebooks. For a jazz chart, writing 7alt is quicker and easier to read than, say, 7(#5 b9). It also gives the pianist or guitarist flexibility to use some creativity choosing which combination to play. Soloists, too. If I were stuck on a desert island with only one chord to jam over, 7alt is probably the one I would pick. It's a fun chord.

BIAB supports 7alt. You can write B7alt and it will work fine.

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