I_dapp is the monthly reward for a DApp for every 1% of the DApp delegation.
Example:
I_dapp = 1,000 ICX
100 ICX is delegated towards all DApps, and DApp_A has 50 ICX delegated to it (50% of the DApp delegation)
DApp_A receives 50,000 ICX per month (1,000 ICX x 50)