Question: How did WHO determine their mask use advice?

Answer passage (source: WHO):
page_title: Q&A: Masks and COVID-19
section_headers: What is WHO recommending to countries that are considering public mask use in community?
section_content: For countries that are currently considering the use of masks, WHO advises
decision makers to apply a risk-based approach and consider the following:
Purpose of mask use: if the intention is preventing the wearer
transmitting infection to others (that is source control) or to offer
protection to the wearer against infection (that is prevention).
Risk of exposure to COVID-19

due to epidemiology/number of cases in the population: if there is high community transmission and other measures such as contact tracing or ability to carry out testing are not possible. 
depending on occupation: e.g., individuals working in close contact with public (e.g., community health worker, cashier). 

Vulnerability of the mask wearer/population: for example, if supplies are
adequate medical masks could be used by people with comorbidities, such as
cardiovascular disease or diabetes mellitus, older people, immunocompromised
patients.
Setting in which the population lives: settings with high population
density (e.g. refugee camps, those living in cramped conditions) and settings
where individuals are unable to keep a safe distance (e.g. crowded buses or
other transport).
Feasibility: availability and costs of masks, access to clean water to
wash non-medical masks, and ability of mask wearers to tolerate adverse
effects of wearing a mask.
Type of mask: medical mask versus non-medical mask (prioritization of
medical masks for health care providers, symptomatic people and their
caregivers).
In addition to these factors, potential advantages of the use of masks by
healthy people in the community setting include reducing potential exposure
risk from an infected person during the ‘pre-symptomatic’ period or if an
infected person is asymptomatic.
There are potential risks and disadvantages that should be taken into account
in any decision-making process on the use of masks:

Non-medical or cloth masks could increase potential for COVID-19 to infect a person if the mask is contaminated by dirty hands and touched often, or kept on other parts of the face or head and then placed back over the mouth and nose 
Depending on the type of mask used, could cause difficulty in breathing 
They can lead to facial skin breakdown 
They can lead to difficulty with communicating clearly 
They can be uncomfortable to wear 
It is possible that mask use, with unclear benefits, could create a false sense of security in the wearer, leading to diminished practice of recognized beneficial preventive measures such as physical distancing and hand hygiene. 


Reviewer feedback on this answer:
This not only outlines the question but give advice on how to wear and how to take care of your face
The answer goes into a great amount of detail concerning how the WHO determines mask rules. It talks about feasibility, types of masks, and vulnerable populations.
there was no relation to WHO but the use of mask and the precautions were well stated

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