Frank Szmulowicz, Ph. D. Dec 11, 2019:

I wonder whether the barrier is the mesenchymal tissue.

Frank Szmulowicz, Ph. D. Dec 11, 2019:

Major Operation/ Surgery is any invasive operative procedure in which a more
extensive resection is performed, e.g. a body cavity is entered, organs are
removed, or normal anatomy is altered. In general, if a mesenchymal barrier is
opened (pleural cavity, peritoneum, meninges), the surgery is considered
major. All surgical departments’ need a well equipped Operation Theater (OT) to
work with whereas Minor surgery is any invasive operative procedure in which
only skin or mucus membranes and connective tissue is rejected e.g. vascular
cut down for catheter placement, implanting pumps in subcutaneous tissue.

As the major operation dissects the mesenchymal barrier, thus the pre and post care and support of the patient is crucial, thus the infrastructure for the patient
and the escorts with required facility is important.
https://pc.odisha.gov.in/Download/2.Major_Operations.pdf

Frank Szmulowicz, Ph. D. Dec 10, 2019:

Major surgery is any invasive operative procedure in which a more extensive resection is performed, e.g. a body cavity is entered, organs are removed, or normal anatomy is altered. In general, if a mesenchymal barrier is opened (pleural cavity, peritoneum, meninges), the surgery is considered major.
http://www.dar.emory.edu/vetcare/surg_definitions.php