NetSuite Development Accounts
NetSuite development accounts are isolated from your production account. You can use NetSuite development accounts to develop and test new apps and customizations without affecting your production account.
It's best to start using development accounts when you begin a new project. It can be difficult to migrate to a development account if you are already in the middle of a development project in your sandbox. You should finish existing projects in your sandbox, and start new projects in a development account.
Note:
You need multiple sandbox accounts if you have multiple large projects which need distinct accounts for User Acceptance Testing (UAT).