Martin Joos' Five Levels of Formality 1962

Level 1: Frozen Levels - Set phrases or conventional structures 
For example : Wedding vows - https://www.theknot.com/content/traditional-wedding-vows-from-various-religions
 

Level 2: Formal Level - Speaker and audience, speech, lecture, no ususal interaction or interruption. 
For example: Wedding speech - http://www.hitched.co.uk/wedding-speeches/example_best-man_speeches_1/adam_3013.htm?selection=Best
 

Level 3: Consultative Level - where interaction is the norm between anyone. 
For example: Tutor and student - https://goodreadingfor.blogspot.co.uk/2013/10/a-dialogue-between-teacher-and-student_12.html
 

Level 4: Casual level - Informal setting between who know each other well, colloquialisms, interruptions. 
For example: work collegues - https://www.easypacelearning.com/all-lessons/english-lessons-level-3/1238-conversation-about-promotion-at-work-between-2-colleagues
 

Level 5: Intimate level - reserved for close friends and family in private, may have code words or shared context.  
For example: husband and wives - http://lqcollections.blogspot.co.uk/2013/04/husband-wife-funny-conversation-must.html