In general, what the others have said. Rules vary widely...real widely. Some shoots allow almost anything, some are more restrictive.
The best way to learn the rules of a given shoot is to look closely at their flier. Rules are usually listed, in some detail, on the flier.
The alternative is to contact the club contact person for an explanation of targets and rules.
In this area, rules are usually limited to round ball loaded from the pouch, no shaders or chew straps, and slings cannot be used for support.
Some others shoots limit adjustable sights. Others, on the extreme end of the spectrum, are flintlock only using only historically correct gear to include any and all items carried on the person...and there is someone to inspect each contestant. However, these PC events are the exception, rather than the rule.
The only rule of thumb I can offer is to expect anything for targets. I have shot chain suspended both horizontally and vertically. Coil springs off of a truck, split ball, cutting a ball and breaking clay birds on either side, charcoal briquettes suspended on strings, acorns suspended on string, candles, feathers, soft drink straws, matchsticks, toothpicks, tacks, metal silhouettes, and rarely paper.
Some of those shots were timed with various means. Rarely with a stopwatch. Usually with an hour glass style egg timer placed where you can see it, or a rattle box.
The most psychologically challenging target I have ever shot is a raw egg suspended on a string at 25 yards, shot off hand. The choice to shoot the egg is given to the competitor. Hitting the egg resulted in double points. Missing the egg resulted in a loss of double points...unless the shooter ate a raw egg kept in a cooler next to the scorer. If the competitor kept the egg down long enough to walk away, he did not receive a deduction of points.
The most physically difficult target required sitting in a saddle mounted on a 55 gal drum which in turn, was mounted on four heavy duty springs welded to the four corners of a steel frame. The "horse" was turned so's the shooter is facing away from the target. The shooter started with a loaded, unprimed gun, turned in the saddle to fire. Reloading was done "in the saddle" and a second shot made on the target, to the rear.
The moral of this long ramble is, expect anything and have fun.