Hi, Dinesh Kafle, then we only need to add the corresponding tag? And what about the field group? what do I have to fill in?

Hi, Jorge Perez. In order to reflect transactions to Cashflow statement you only need to add Tags to the chart of account. Groups in odoo can be used to as a parent of ledgers to consolidate the amount of ledgers in financial statements such as Trial balance with option 'Hierarchy with Subtotals'. You can create groups as you like but you need to be careful to add a account code while creating the ledgers (CoA).
Example : if you have two Ledgers 151000 Land and Building and 152000 Furniture and Fixture then you can group them to one group as Fixed Assets by creating new group Fixed Assets with Code from 151000 to 152000 and can see the consolidated balance of these ledgers in Trial Balance with Hierarchy and Subtotals.
Note: I have used this feature in Odoo 16 Enterprise