What's the difference of the base between XVBC21 and XVBC07?

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In our catalog, XVBC21 is written as 'For use with Modular tower lights without "flash" discharge tube unit.
And XVBC07 is written as 'For use with Modular tower lights with "flash" discharge tube unit'.
'without flash' means modular lighting unit, like XVBC4B4,XVBC4B5,...
'with flash' means beacon unit which doesn't need top cover, like XVBC1.,XVBC6.,..
In fact, XVBC21 = XVBC07(Base) + XVBC081(Cover).