Deep cycle lead acid batteries have think lead plates as compared to normal lead acid batteries with thin plates and large surface as produced in market for high cranking voltages as recommended for vehicles. Normal lead acid batteries can only be discharged to 80% of its capacity (10.3 Volts) before requiring to be charged again, if you will go below this then you will be damaging them.
None of the companies here in Pakistan clearly quotes their batteries as deep cycle batteries as otherwise it can be discharged to around 6.5 volts before requiring to be charged again. Explanation says sulfate crystals forms quickly on thin lead plates completely destroying it whereas think lead plates resists the development of these crystals to a more extent and charging again would restore it if ever it develops. You may further search for why it is the case as I may not be 100% right in explaining the reason as its been a while that I read it.
Pheonix ugs grade series is specifically developed for ups application, has little think plates but sites never clearly says it as deep cycle batteries otherwise if it says that we can easily adjust the low cut off voltage of around 6.5 Volts.
As far as the second point is concerned that flush is more suitable in bigger batteries because mostly bigger batteries (used in UPS) dies because of gunk gathered at bottom which forms a conductive layer whereas in smaller batteries installed in car, its mostly dies because of remaining in undercharged state therefore flush is not very helpful here.
