Rule 15.1 states that players must observe the Rules and the spirit of the game. One of the unique characteristics of squash is the demand for sportsmanship between the 2 players who essentially share the same court space. Often people feel that if there is contact of any kind between the 2 players, then a let (or a stroke) is warranted.
This is not the case, rule 12 sites "Interference", and although the player whose turn it is to play the ball is entitled to freedom from interference by the opponent. Discretionary calls are emphasized in 12.2.3 "reasonable swing", 12.7.1 "Minimal Interference" and 12.7.2 "Effort to play the ball".
A reasonable swing means, if I am behind you and you have to play the ball in front of you and you contact me with your racket, it is a NO LET because you have an excessive backswing(rule 12.10)(an excessive backswing is one in which a player's arm is extended towards a straight arm position and/or the racket is extended with the shaft approximately horizontal). But if I am behind you and you have to play that ball where you stand or behind you and you hit me with your racket then, it's a STROKE. Playing a LET because of an unreasonably large swing is often called in these situations, when the truth is it is up to the referee to use their discretion and make the call of either a STROKE or a NO LET.
Bumping is a common problem at all levels of squash, but just because there was contact does not entitle a player to a LET. If players stopped play everytime there was any contact, games would never end. This is often a tactic of a lazy player who may feel that the ball has aleady past them and it would be easier to ask for a let. It is up to the discretion of the referee to decide if contact has indeed interupted the players ability to play the ball. Too many times, referees reward these lazy players with a LET when in fact, it is a NO LET.
If interference occured, but there was no way you were gonna get to the ball, NO LET. If interference occured but you were making no effort to play the ball, NO LET. It's up to the discretion of the referee to decide if the player made every effort to play the ball.
Rule 17, Conduct on court - deliberate contact and dangerous play. Often players who try to use the LET and STROKE rules to their advantage often neglect Rules 12.12.1 and 18.104.22.168. Initiating contact to ask for a LET, running directly into an opponent instead of playing the ball or hitting the ball to attempt to hit the opponent with the ball to get a stroke should result in a CONDUCT WARNING, then a CONDUCT STROKE, then a CONDUCT GAME then a CONDUCT MATCH awarded to the opponent after multiple offences.
There are sooooo many discretionary calls in squash, I myself as a player feel that I need to trust that my opponent will be a fair player and that the referee will not try to include themselves in the match but rather provide correct calls on the play. But this is often not the case, people don't call their double bounces, or their scoops, and nobody thinks they hit the ball out of bounds. But referees too many times either over-officiate cause they think they know the rules, or under-officiate cause they have no idea.
The lesson is read the freakin' rule book (RULE 15.1) before you play or referee a match.