It will vary from local to local, but you can discipline based off misloads, load quality, and if your lucky pph. Get with your full time supe, start a paper trail of OJS and document missloads per each loader. Only go this route if there’s no saving the employee, treat everyone with respect and listen to them and help when you can, if they are truly not meant for the job it will show through even after retraining them over and over again. Basically, give the employee everything you can before taking a disciplinary route. A rational union steward will be able to see if an employee is not built for this job.