Periodization is simply training different attributes of a given sport in different phases. This is usually done because of necessity. You are confused because:
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You are looking at this from a Bottom-up approach instead of Top-down. You are unable to define the pertinent attributes of your sport which actually need periodizing and why they must be periodized. I don’t blame you for this. (Strength vs hypertrophy… bleh!)
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The complexity of a periodization plan will depend on the sport itself. For example, a yearly powerlifting plan for non-elite lifters may have considerable overlap and less phases as opposed to an Olympic lifting plan.
Non-linear and Undulating Periodization are… I don’t know man… I basically call the former a progression model since the aspects of programs like 531 apart from the main lifts do not change with the different “phases” other than the deload week, and the latter “heavy, medium and light” days.
I suppose you could justify calling the latter “periodization” by your intent when you execute the lifts, eg, adapting to maximal weights on the power days, skill training and maximal acceleration with submaximal weights on strength days and err…more sets with higher reps for less stress on the joints on hypertrophy days(note: all these also relevant to points 1 and 2), instead of simply separating them by rep ranges. This is why the confusion occurs since the overlap is immense and people start thinking low reps for strength and high reps for hypertrophy is the only defining factor when training these attributes.
Conjugate periodization? I only conjugate with lions. (Joke. I don’t have enough knowledge about this system to comment.)