arpadveress 
Shall is to be understood as mandatory. Deviating from a “shall” statement requires derogation approved by Company.
Should is to be understood as strongly recommended to comply with the requirements of the specification. Alternatives shall provide a similar level of protection and this shall be documented.
May is to be understood as permission.
Can is to be understood as a physical possibility.
Must expresses a regulatory obligation.
All above may help how to ready standards, regulation, or similar.
Keep up good work
