As FramerDave eluded in post #2, "mounting" refers to all kinds of attachment inside the frame. Some mounting techniques are more durable, more reversible, more permanent, more risky, more costly, more labor-intensive than others. The best strategy, IMHO, is to become proficient with all kinds of mounting, acquire the equipment for as many techniques as you can, and work with full knowledge of what you are doing. Generally, use the least-risky methods and avoid permanent changes to items of value. Never experiment on customers' property.
Every framer decides his/her own policy on mounting. Some framers dry mount everything unless there is a reason not to, just because that technique is generally effective and well within their comfort zone. Personally, I prefer to use the least-invasive, most reversible mounting technique that will hold the art securely. These days, most items in my shop are mounted using no adhesives in direct contact. Edge supports and platform mounting are my favorite techniques for paper items in good condition.
If you must use a permanent mounting method on a porous-surfaced item, wet-mounting under vacuum is still the most durable and cheapest method. Dry mounting is probably easier and almost as durable, if done properly. Pressure-sensitive adhesives are less durable, and spray adhesives are the least durable and most costly. It is a curious fact that generally, the best mounting methods use the cheapest materials, and the worst methods use the most expensive materials.
The choice of substrate is also important. 100% cotton paperboard, 4-ply, 8-ply, or more ply is suitable for most mounting methods, cost-effective, durable, non-invasive, and easy to work with. If you have a vacuum press, you can use starch paste to laminate as many 4-ply layers as necessary for the size and rigidity required. Virgin alpha cellulose matboard is next-best; still suitable for all levels of preservation, and costs less. Warping is a problem if too much moisture penetrates one side. You can use fluted polypropylene to reinforce a single 4-ply mounting board, which provides good rigidity, warp-resistance, and low cost.
Foam board is cheaper, more commonly used, more resistant to warping, and suitable for most retail framing. However, conservators still hesitate to recommend it, due to concerns of the expanded polystyrene core's chemistry.
PVC sheeting, such as Sintra brand, is smooth, warp-resistant, easy to work with, and suitable for decorative-only framing. Trouble is, it is cemically invasive, especially in the event of elevated temperatures or humidity. It reacts badly to heat, so dry mounting is out of the question. And it is non-porous, so most water-based adhesives will not stick to it.
Aluminum composite material (ACM), such as DiBond and ePanel, is an excellent substrate for dry mounting. Its downside is that most framers do not have a Fletcher FSC or KeenCut machine to cut ACM with clean, rounded edges.
Wood and wood-composite boards, such as MDF, hardboard, plywood, and particle board have no applications in my shop.