To execute a project, following 2 engagement models are most preferred:
1. Hourly/Monthly Billable Model: Billing is done on the hourly or monthly basis as resource utilized. Time is tracked using most advanced Time & Activity Tracking Tools and client can view resource activity & performance on real-time. This model is adopted mostly for Agile Development Methodology.
2. Development Center Model: A dedicated team with multiple resources are assigned for a certain time period and billing is done for the team for that specific period. In this model, invoice is raised on different milestones where 1st milestone invoice is paid in advance.