The Balances pallet example of storing the balance of an account.

Example

 impl pallet_balances::Config for Runtime {
   type AccountStore = StorageMapShim, frame_system::Provider, AccountId, Self::AccountData>
 }
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You can also store the balance of an account in the System pallet.

Example

 impl pallet_balances::Config for Runtime {
  type AccountStore = System
 }
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But this comes with tradeoffs, storing account balances in the system pallet stores frame_system data alongside the account data contrary to storing account balances in the Balances pallet, which uses a StorageMap to store balances data only. NOTE: This is only used in the case that this pallet is used to store balances.