ICD-10 Uses Alpha numeric characters.
The first character is always a letter A-Z with the exception of U
The second character is always a Number 0-9
Characters 3-7 can be either a number or a letter (M1A.- or O9A.- , or S82.-)
The first 3 characters define the category, the decimal is there to remind you that characters 4-7 are an extension of the category.
Not all codes are 7 characters and not all 7th characters are A, D, or S. Some are numbers such as the 7th characters for the coma scale.
each place after the decimal has a meaning for the category such as
T23 is the category for burn or wrist and hand
the fourth character represents the degree of burn
the 5th character represents the specific site of the hand
the 6th character represents the laterality
the 7th character represents which encounter this is
so for a third degree burn of multiple sites of the left hand the initial encounter
T23.392A
For the poisonins
6th character matches up to the 6 columns in the table of drugs and chemicals, some of the codes have a 5th character and some do not so there is a placeholder x for those that do not have that specific characteristic
so adverse effect of penicillin initial encounter: T36.0x5A
And some codes need multiple placeholders such as bitten by dog
which has no valid 5th or 6th character but a required 7th for which encounter it is
W54.0xxA
or contact with broken glass which has no 4th, 5th, or 6th but a mandatory 7th
W25.
xxxA
As far as the R goes, R codes are symptom codes, S codes are fractures, O codes are obstetrics, M codes are musculoskeletal and so on, the correlation of the alpha to a body system is purely coincidental.
I hope this helps you (I teach ICD-10 CM)
T36.0x5A