Its up to the union to determine rules like seniority. My union has no seniority - if you suck, you don't get hired. if you break a major job rule, you'll get fired.

Union bonus is, that when your a-hole boss has a grudge and fires you for a bogus reason, you can dispute it, and possibly get your job back too.

Our contracts also spell out specific job titles and positions, and pay rates accordingly - a guy who only sweeps gets $x, where as a special effects technician gets nearly 3 times that amount.