I paid and got service from dapp. does dapp wait until enough confirmation is settled? or not, is it possible that tx can be reverted?
First a bit of background. Here are some of the things that can happen when you send a transaction:
1) The transaction may or may not be included in a block soon. This depends on network congestion and your used gas price. Especially with zero gas price the tx may never be included in a block. Furthermore the miner who includes the tx in his block has to be able to solve the PoW mining "puzzle" to make the block complete. If the tx is not in a completed block the tx is not counted as valid - it will probably be included in another block.
2) When some miner includes the tx in his block and manages to solve the PoW puzzle the block is considered complete. A complete block is added to the blockchain. But that block may still become an uncle block which would again make the block invalid and all transaction in it are again available to be added to other blocks. If a block is an uncle block it is not considered to be part of the canonical chain - the chain which all blockchain explorers show.
In theory any block anywhere in time can still become an uncle block. But the more time passes the less probably that is. This is what confirmations are for - after for example 10 blocks (10 confirmations) it is very unlikely that a block would become an uncle block so most services (for example exchanges) consider a transaction as final after for example 10 confirmations.
As for your original question: the dapp creator decides on how it works. Most likely some amount of confirmations are required. But in any case as long as the tx doesn't result in some invalid operations (and gas price is above 0) the tx will be included in the canonical chain at some point.