What's the main difference between Baratoplata and Kimura lock? Or is Baratoplata another variation of Kimura? Do they affect to the same area of the arm?
The kimura, omoplata, and barataplata are all attacks on the shoulder. The end result is the same but they apply the attack in different ways.
I'll do my best to describe each attack below, but these descriptions would make a lot more sense if you accompany each one with an image of the attack (probably just a quick google image search would be good enough).
The end result is to isolate the tori's (receiver's) shoulder joint, immobilize tori's elbow, and move the tori's wrist along a path that would circle it towards the tori's shoulder blade. A similar effect is achieved if tori's wrist and tori's elbow are circled in directions opposite to each other.
To apply the kimura, the uke (attacker) uses a figure-four grip of their arms to accomplish this, where one of the uke's forearms blocks the elbow from moving while uke's other hand controls the wrist of tori and moves it in a circle towards tori's shoulder blade.
The omoplata accomplishes this using uke's legs. Tori's bicep & tricep is trapped between uke's legs so that the legs have isolated the shoulder and immobilized the elbow. Then, uke lifts his hips into tori's forearm to circle tori's wrist towards the shoulder blade. (The name "omoplata" comes from the Portuguese for "shoulder blade".)
The barataplata uses a combination of the arms and legs. Generally, only tori's elbow needs to be trapped between uke's legs. Uke's arm threads through tori's elbow and uke places his hand on his own thigh in order to prevent tori from withdrawing his arm. Then, a rotation of uke's body and lifting of uke's arm (the one that threads through tori's arm) like a lever is what circles tori's wrist one way while circling tori's elbow the opposite way.
The name "barataplata" is a portmanteau of "Barata" (the nickname of the jiu-jiteiro that invented the move, Rafael "Barata" Freitas) and "omoplata". Barata + omoplata = barataplata. I've also heard barataplata called the "arm-out omoplata" because it is very similar to the omoplata (i.e., a shoulder attack using the legs) except much of the tori's arm is outside the grip of uke's legs.
Baratoplata and Kimura hurts the same joints and area Shoulder. It means both submissions are to same result but with different variations.
In the same way of the Arm bar/arm lock. This submission hurts the elbow and could be apply for guard, mount, back, inverse and also from inside the my opponent guard (not usual)
The history of Kimura:
The kimura lock, also called double wristlock (catch wrestling), chicken wing or gyaku ude-garami (judo) is a grappling submission hold of uncertain origin, being catch wrestling and judo the likely authors of the position. In Brazilian jiu jitsu the submission bares the name of “Kimura” after the famous Masahiko Kimura who defeated Helio Gracie with this lock on the 23rd of October, 1951. The submission relies on isolating the shoulder and elbow joints with the use of a double wrist grip which allows for the attacker to apply leverage against both of these joints (though mainly the shoulder).