IFAK literally stands for Individual First Aid Kit. There are many different types of first aid kits and they should be designed around your purpose, whether that be hiking, camping, hunting, fishing, or just driving to and from work. Acronyms have become all the rage these days so don’t let the fact that a first aid kit is just that, a first aid kit. First aid kits come in all different shapes and sizes ranging from the MFAK, Mini First Aid Kit all the way up to full EMS Trauma bags. For the purpose of this post, I will be focusing on various IFAKs.
Now I am not a Combat Medic nor am I in the medical field, so many of the these I will be referencing withing this post I HIGHLY advise getting the proper training before using. I am however someone who understands the importance of being prepared, which includes the necessary training needed to use the items that are contained in your first aid kit.
Now, while every IFAK should be purpose-built, there are a few things that should be in every IFAK no matter what. Those items are as follows.

ESSENTIALS

Bag ( Obviously! )
Tourniquet ( CAT, RATS, or SWAT )
Clotting Aid
Gloves
Gauze Roll
Surgical Tape
Pen Light / Small Flashlight
Small Sharpie
and
EMT/Trauma Shears
These are the starter items that are a must in every kit as these can be used for pretty much any scenario you could possibly encounter. Tourniquets are important as they are used to stop bleeding on the extremities. Clotting Aid is used for center mass wounds or wounds that aren’t able to be stopped with a tourniquet. Gloves help prevent contracting or passing infections and disease. Gauze and tape will be used to create bandages, which are vital in keeping the wounds from becoming infected and also help with bleeding. The Trauma Sheers are there to cut clothing, seat belts, or whatever else you may need to remove to gain access to the affected area.
Some of the more advanced items you can add to your kit, which I highly recommend gaining real training on before using on yourself or someone else is as follows.

PROFESSIONAL GRADE ADD-ONS

Nasopharyngeal Airway
ARS Needle for Decompression
SAM Splint
From here is where you can start to customize your kit based on the type of activity you are preparing for. Obviously, you could build a big kit that has everything I am about to mention, but for general first aid, these are the items I tend to keep in my personal IFAKs. Some items may be duplicated in these kits.

GENERAL ( Contained in all following kits )

Green Chem Light
Duct Tape
Survival Blanket
PES Eye Shield
ETD Responder 6in
CPR Face Mask
Tweezers
Neosporin

CAMPING

Multiple Sized Bandaids
Wound Closure Strips
Anti Burn Cream
Wound Cleanser Bottle
Syringe
Thermometer

HUNTING

Eye Wash
Elastic Bandage
SAM Splint
Water Proof Wrap
Small Sharp Knife
Self Grip Wrap
Water
Wound Irrigation Shield
Gauze Pads
Square Bandages
Hyfin Vent Chest Seal

FISHING

Small Needle Nose Pliers
Water
Wound Irrigation Shield
Water Proof Wrap
Eye Wash
Duct Tape
Extra Burn Cream
Zip Lock Bags (Multi-Use Item)
These are just the basics and sometimes things will get added updated and /or removed from the kit. For me, I am usually not far enough away from civilization to need what I would refer to as an expanded kit. This would be a kit that contains enough materials for your entire party, along with items from every kit to account for every potential issue. Extended kits are great for backpacking trips and hunting trips that require you to be in more remote areas.
A few good resources if you are interested in purchasing a prebuilt IFAK kit or just looking for quality gear are below.

https://www.narescue.com/
https://www.rescue-essentials.com/
https://t-medical.org/
https://www.tacmedsolutions.com/
RATS Tourniquet