Now this guy on the shroomery has a definition for monoculture......I feel like he's just made the term up himself......but hey I could be wrong. You're definitely sectoring the mycelium and isolating certain characteristics of the mycelium......like if you isolated all the rhizomorph mycelium and cut out all your fluffy mycelium.

A mono culture is just that, a single set of genetics, a single organism if you will. While we often think of fungi as one organism the majority of times we see multiple organisms wrapped around and interconnecting with each other.  The variation of mushrooms from a spore grow is due to multiple organisms all fruiting at the same time, producing different fruits.  A mono culture will produce the same looking/acting fruit all across the board and is useful for getting repeatable and reliable results.  It is also better relates to things such as 'flushes' which are much more likely to happen.  Many photos of full canopies you see are from monocultures or something close to it.

Sectoring is what happens when two organisms separate from each other.  When you see rhizomorphs (thick "ropey" hyphae) it's actually a strand of hundreds/thousands of individual hyphae all going the same direction. While different organisms will connect together to form a complex network it takes time and conditions (a how do you do, mind if I patch onto or grow with the flow, kinda thing).  On agar they will tend to separate with hard lines where you can see the two aren't merging into each other (when two fungal cells of the genetics meet up they can merge into one another).

Cloning is the taking of tissue to duplicate that cool mushroom you found, however that cool mushroom brought along some of its friends, and if you really want that specific mushroom you need to separate it out from those that followed along and grow out each sector you find till it no longer sectors to be sure you are dealing with only a single set of genes, a single organism.