The two formats you will meet
Almost every amateur event uses one of these two formats, or a mix of both. Round robin means everyone in a group plays everyone else. Knockout means lose and you are out. That difference decides everything, from how many matches you will play to how you should treat the first point.
Round robin: everyone plays everyone
Players are split into groups, usually of four to six, and each plays every other member of the group. The combined results decide the standings, so in a group of four you play three matches whatever happens.
Standings are usually decided by matches won first. If that is level, finer measures come in: the head-to-head result, the sets, or games won against games lost. The details vary from event to event, and they are worth reading for the one you are entering, before it starts.
The advantages are plain. Several matches, a range of styles, and a first-match loss does not end your day. Players new to competition tend to get the most from this format, because it offers both experience and a second chance on the same day.
The limit is time. A group of six is fifteen matches, so organisers often use short sets or a single set to finish in a day, and a group's last match can sometimes no longer affect the standings, which takes the edge off it.
Knockout: lose and you are done
Players are drawn into a bracket: win and you go through, lose and you are out. Far fewer matches are needed, a draw of sixteen takes just fifteen to find a winner, so organisers can afford full sets more often.
Many events seed the draw: the stronger players are placed on opposite sides so they cannot meet in the first round. An unseeded player may therefore face the toughest opponent in the event in their very first match. That is the nature of the format, not bad luck.
The strength is that every match counts. There is no match played for nothing, and there is one clear winner. The weakness is that half the field plays a single match and goes home; for someone who travelled a long way for a first tournament, that can be a very short day.
Some events answer this with a consolation draw: first-round losers play on in a second bracket, so everyone gets at least two matches. If the event you are entering has one, the pressure on the first match drops a great deal.
The hybrid
Many well-run events use both. A round-robin group stage gives everyone several matches, and the top of each group then goes into a knockout to find the winner. It gives you both the match count and a clear ending, but it needs the most time and usually runs over more than one day.
How the format changes the way you play
In a round robin, losing the first match is not a disaster, so it is a good time to play the way you have been practising and to learn something about each opponent. In a knockout, the first point of the first game weighs as much as the last; players who start slowly are at a disadvantage, and warming up properly before you walk on matters more in this format.
The gaps between matches differ too. A round robin has predictable waits; a knockout can leave you waiting a long time while another pair plays a long set. The food and water in your bag matter as much as the racquet.
What to ask the organiser before entering
Three questions are enough. One: the minimum number of matches you are guaranteed. Two: how long each match is, full sets, short sets, or a tiebreak in place of a final set. Three: when the event expects to finish. Those three answers tell you what the day will be like better than the format's name does.
An event that can answer all three clearly before you enter tends to be well run on the day. One that cannot is one to enter expecting the schedule to slip.
Choose the event that gives you what you want
If the aim is to play as much as possible and gather experience, choose round robin. If you want to find out how you handle pressure, choose knockout. And if you have the choice, a hybrid gives you both.
THE SET.'s sessions state the format in each session's details: the type, the number of sets and the approximate running time. Sign up and choose the session that matches what you want from the day.
General guidance, not a research finding.