Business Developer Full stack - he/she is the first sales hire when the CEO has no more time to sell. Why “full-stack” ? He/she will manage the sales cycle from the beginning (prospection) to the end (account management).
Sales Development Representative - Manages lead generation outbound/prospection. He/she books meetings for the Account Executive. The Business Developer Full-Stack can thereforce focus on closing and account management.
Customer Success Manager - You should recruit one when the number of clients is so important it can not be managed by the Business Developer alone anymore.
The Business Developer Full Stack becomes an Account Executive. His/her main role: close leads sent by the SDR + handoff to customer success after closing.