Some sort of pathogen sorry, It is the General Instruction for the Roman Missal.

They are a set of instructions that tell us how to say the Mass (OF), the EF have equivalents in 'de Defectibus' and the General Rubrics (various editions).

All copies of the Missal for use by a priest should have the Instructions at the front. The real dumb question is "Why do many clergy not know what the instructions are?" One has the impression that they have read them, think they know how to say Mass, and have never bothered to look at them again. The instructions changed in some details with the 2010 edition, for example about cleaning(purifying) the sacred vessels after communion, in my limited experience few English priests follow these instructions accurately.
Note that some of the details are determined by the Conference of Bishops (who may delegate some to the Diocesan Ordinary), and there are differences, about music and posture, you need the local edition. Note also that the instructions are not exhaustive, there are older guidelines which still apply in part, and notices interpreting GIRM put out by CDWDS (look it up). That it why the Archbishop of Portland still finds it useful to issues diocesan instructions.
I sometimes mention on this forum IGMR, the original text in Latin, from which these are adapted.

The GIRM is also available on the web in various places. In some countries, there are slight modifications from the standard edition, approved by the bishops conference and the Holy See.

For US readers: http://www.usccb.org/prayer-and-worship/the-mass/general-instruction-of-the-roman-missal/
For Canadian readers: http://nlo.cccb.ca/images/stories/GIRM.pdf
For readers in England and Wales: https://www.liturgyoffice.org.uk/Resources/GIRM/Documents/GIRM.pdf
For Australia: https://www.catholic.org.au/commission-documents/bishops-commission-for-liturgy-1/164-vatican-approves-general-instruction-of-the-roman-missal-for-australia-1/file
For Ireland: http://www.liturgy-ireland.ie/uploads/8/4/2/9/8429650/roman_missal_girm-ibc_2011.pdf