It typically means they’re wanting to buy the rights to future royalties of just that well, and not the minerals interest. You would have rights to any new wells drilled in the minerals, and their interest would cease when the well is plugged. It’s the least amount of risk an investor will ever see in the industry. Are you leased and receiving royalties, or do you have working interest in this well?
In Texas, assigning interest in a wellbore means the new owner will have rights to anything that well produced from that wellbore.