In VRLA batteries the hydrogen generated at negative plates is mixed with oxygen generated at positive plates and turned back to water thus avoiding the need to replenish distilled water to electrolyte like in open vented lead acid batteries.
VRLA batteries have a pressure release mechanism to avoid rupture during charging at high voltage.
The biggest advantage of VRLA batteries over normal lead acid battery is no spillage in any position.
Since lead acid battery contains acid transport regulation classifies batteries under Class 8 (Corrosive substances).
Entries in IMDG Code for Lead acid batteries