CNC – Computer Numerical Control. CNC-machine has numerically computer controlled axes that move tools and stock to make a product. CAM (Computer Aided Manufacturing) software is required to generate those numbers for every part. 

The CNC machining center is an advanced manufacturing machine, a robotic tool that can perform a variety of machining operations with high precision, high quality and high-detailed surface finish. A CNC machining center can perform drilling, milling, tapping and many other operations.

Many ordinary things around you. Most plastic products have CNC-machined injection-mold tools to make them. Many phones and tablets have CNC-machined aluminium housing. 

Production max volume X 500.0 mm, Y 400.0 mm, Z 330.0 mm for 3-axis machining.

4- and 5-axis machining have smaller volume.

Requirements are mostly:
3D model must be made for 3, 4 or 5 axis machining;
Inner corner radiuses minimum 1.1mm (smaller radiuses available on request, requires special tools);
Model must be optimised for high-speed machining;

Most experienced solid 3D modelers should be able to prepare machinable 3d part model.

By experience, knowledge, and best guess usually.
When we have placed an estimate, and it will be accepted, we will take commitment to deliver.
To get the estimate, things must be prepared in order. 

Digimachining needs to prepare CAM-files for specific robot and tools for every 3D model. Model itself has to be machineable, so it it depends on 3D modeler experience and professionalism how much engineering services are required (if at all) to make part 3d machineable. 

Yes, that can be normal. Probably your 3d model need some serious tweaking before it comes machinable or practical to make.

Yes. CNC-machining is commonly used to make high-precision surface finishing for 3d-printed hollow comb titanium printouts, for example.