In Division I, the NCAA stipulates that no more than 3 coaches can be paid from the university's typical funding source for providing coaches' compensation. As a result, programs tend to have a Head Coach, an assistant who teaches hitting as a primary responsibility, and an assistant who concentrates mainly upon pitching paid from that source as their principal source of compensation.
The "third assistant" or "volunteer assistant" as it's often called, is paid from other sources available to the program. Typical sources include camp revenues, corporate sponsorship (i.e. equipment contracts), and radio/TV shows. It's also not unusual for third assistants to serve as paid clinic/camp instructors and/or summer team coaches when their college duties don't conflict.
As you might imagine, the compensation associated with these third assistantships can vary quite a bit from program to program; depending upon the size and number of non-university revenue sources associated with each program. However, they are invariably full-time, paid assistantships; causing the term "volunteer assistant" to be a misnomer. Because of this, more and more programs are dropping the "volunteer" part and referring to this person simply as an "assistant coach."
As full-time assistants, they are usually asked to be involved in a wide range of coaching responsibilities; although, their background will tend to cause them to spend more time on either the hitting or pitching side of the coaching to be done.
More often than not, they are given a full seat at the coaching table. However, there is one enforced distinction from the NCAA: They usually cannot travel expressly for recruiting purposes. I say "usually" because the actual regulation stipulates that no more than 3 coaches may be designated as traveling recruiters. However, they can be actively engaged in correspondence with recruits (subject to other general restrictions, of course), and they can recruit when on their own campus.
Given all of this, the "third assistant's" position is universally viewed among college coaches as THE place to begin one's coaching career; and virtually every coach will tell you they learned a ton during their time in that role. Because it is viewed in this way, the position is in high demand and can be highly competitive.
The combination of one's experience, track record, and networking tend to be required to do this. As part of the early networking process, I'd recommend that a prospective applicant meet with a number of college coaches, reach out to their prior college/professional coaches, and accept every opportunity to instruct at camps, clinics, and development centers that they can.
For the former player who brings passion, teaching ability, and a dedication to hard work, the role can be a very fulfilling one; one that is virtually a required stepping stone to more responsible coaching positions.
If the OP or anyone else interested in the role continues to have questions, I'm always happy to reply to private messages.