International Swaps and Derivatives Association (ISDA) regulates the variation and initial margin (IM) and standardized it in a model called Standard Initial Margin Model(SIMM).
By far, there are 9 posts about SIMM by ISDA
https://www.isda.org/category/margin/isda-simm/
The most recent model is ISDA SIMM version 2.1 which is effective starting from Dec. 1st, 2018.
https://www.isda.org/2018/08/27/isda-publishes-isda-simm-2-1/
https://www.isda.org/a/zSpEE/ISDA-SIMM-v2.1-PUBLIC.pdf