A field applications engineer is a person who is payed to work directly with the customer's technical staff
for any or all parts of a product's life span, aiding in the design, deployment, and maintenence of that product. To elaborate, a field applications engineer has three primary responsibilities:
On the front-end of a potential buisness deal, a salesperson will often request the aid of a field applications engineer to determine if an organization is capable of deploying its product into a customer's site with any or no modification. The engineer may even participate in the selling process directly, sometimes coming up with a bid if modifications are necessary. These duties are often carred out on-site.
Also on the front-end of a buisness deal, a field applications engineer may be deployed to simply gather data about a customer to determine marketing or engineering requirements for an upcoming or next revision of a product.
If a sales deal was reached, the field applicationsengineer's next responsiblity is to stay in contact with the technical personnel of the customer organization to prepare them for deployment of the product, often by conducting on-site training, and to communicate any of the techincal personnel's concerns to the correct people within the field application engineer's organization.
The final responsibilities of a field application engineer is to deploy the product and be as helpful in the debugging process as possible. Often this means gathering technical data all the while keeping the customer calm and assured. After the (hopefully) successful deployment of the product to the customer's site, the field applications engineer's responibilities often shift to mainting a good technical relationship with that customer: often travelling on-site when standard means of technical support break down.