Jiu Jitsu is universal.

Jiu Jitsu techniques are universal. Some may say that nogi thechniques are more realistic for example and that a cross collar choke is not, but if you opponent is wearing a leather or denim jacket the cross collar will send him to sleep in seconds, on the other  hand its a completely useless technique to know and master if you are only training , preparing and fighting in no gi championships and even though it can indeed save your life even in the streets I am pretty sure you can handle the average Joe with no problem after some level of proficiency on wrestling and grappling . So in that context all Jiu Jitsu can be used in all the situation especially if your opponent is and unskilled layman. Having that being said each style and situation has some very important tips to keep in mind.

Sport Jiu Jitsu

In sport Jiu Jitsu the most important aspect is to know the rules. Know the points, how to score them and how to score advantages. Also a big factor is to know how to stall without looking like you are stalling. This might not sound nice but definately can make a profit when you are rolling in a point system sport jiu jitsu based enviroment.

Submission Only

In sub only situation things are very different. Open guard is a huge tool and can be implemented virtually from anywhere. Having your guard passed or your back taken is not the end of the world as long as you can survive and better your position by the time. Another important aspect is to be dangerous even from the worst positions.  Having some finishes from bottom side control/mount or having your back taken and properly executing right on time is like having an ace in the sleeve. Defending and escaping submissions is major as well, when you defend  a submission you are just winning some time for your next move and your next move is the escape. Very often after a failed submission the previously attacker  finds himself in a place of defending and that is the moment you have to take advantage of.

CJJ/MMA

While training or fighting MMA with a Jiu Jitsu perspective the most important concern is distance management. When standing the safe distances are 1st) two arms distance 2nd) clinch 3)floor. So most important is to be good at your striking and take down wrestling. On the floor(CJJ/MMA),top position is almost always favoring having the ability to pass the dangerous legs and establish a position where you can ground and pound effectively until a submission opportunity appears. There have been several occasions where fighters have been extremely successful fighting MMA from the guard and such examples are Kron Gracie,Brian Ortega, Tony Ferguson as well as legends like Helio and all the early Gracie challenge fighters. The guard can protect the practitioner from punches of the untrained and provide a really useful platform to implement elbow strikes to the head from short distance using your hip to push away opponents and then let them or pull them right to your elbows.

Self Defense 

WARNING!

The best self defense provided by Jiu Jitsu is verbal Jiu Jitsu to deescalate all tension. Avoid conflict and physical contact at all cost as long as you or someone around you is not in danger. If its needed to use Jiu Jitsu for self defense strike first and strike hard (elbows, knees). If the fight goes to the ground try to manage the distance, control the fight and technical stand up to get out of it. If there is a weapon TOTAL priority is control of the weapon arm and disarm. There have been example cases where people tried to choke an attacker having a weapon on him and since his arms were free he used it. Stay safe and avoid street conflict.