We are in the process expanding the TBM practice to all divisions withing our Company.  As a result, we are evaluating/reevaluating our fixed vs variable cost labeling logic. Is there any industry standard documentation on this topic?  We need this designation at a lower level than the GL Account (probably the charge level).  Thanks in advance for any feedback.

We track at GL Account and Vendor level. Not sure how you can get more granular than that without having Procurement/Accounting staff have this understanding as they process PO/Invoice as well as Journal entries for adjustment/correcting entries. 

It would interesting to see if others are using AI/ML to be able to track this type of transaction details and create a logic/rule for it

In my past life as an accounting Controller, all depreciation was fixed as was anything pushing down from the balance sheet and anything with a fixed term of 3 years or greater. Everything else was variable because it was either physical goods and materials so the volumes and costs would flex on sales, or it was based on consumption that was unrelated to sales. Things like labor and utilities flex on consumption.

Since we're talking about management reporting and not competing financial reporting or tax filings based upon this data and processes, it really depends on how your organization defines fixed and variable. You could look into accounting standards like GAAP and IFRS but I don't think you'd find much value in that for this purpose. Anything that costs the same thing all the time regardless of whether you sell something or not is fixed, all else is variable.

As @Hee Sung Kang has said, GL account level is usually where you find this designation. Most often, in my experience, fixed and variable costs are rarely booked to the same GL because fixed costs are related to a longer term and stated as paying down long-tem liabilities which is a different area of the P&L than other OpEx (typically).