First: separate the sale from connectivity
Stellar POS works offline and keeps local checkout operations running when the network connection is interrupted. Scope is configured for the version and installation and verified during technical assessment.
External payments are a separate process
A checkout continuing to operate does not mean that a bank, gateway, external fiscal service or data lookup is available. Each integration has its own communication requirements.
What to validate in a demo
Ask for a test of critical processes during an interruption: sale registration, permitted payments, documents, inventory and synchronization recovery. Ask for the scope to be documented in the proposal.