Answer: Cultivar is short for "cultivated variety" and can mean different things to different people. It may be a selected seed strain, meaning plants are grown from seed collected from a certain population of parent plants that shows certain characteristics. Sometimes these are in fact hybrids, but certainly not always. This form od selection and breeding might be considered engineering, in a very strict definition of the term. Cultivar can also mean it is a named variety selected on the basis of some significant characteristic such as, for example, consistently good fall color. This plant would then be propagated vegetatively and all resulting progeny would be identical to the parent. The propagation might be done by rooting cuttings or by root division, by tissue culture, by grafting, by budding but not by seed because seed introduces the possibility of variation. Again, these means could be considered engineering, depending on your definition.
Generally, selections that make it into the trade for commercial production and sale are made on the basis of garden worthiness, with value being placed on cosmetic or ornamental preferences such as unusual coloring or better flowers or fruit or unusual natural shape or dwarf mature size, or sometimes on more practical considerations such as better disease resistance or a demonstrated wider tolerance to say heat or cold or soil pH than would be found in the straight run species. Generally selections are made to improve on the original in one way or another.
If you are familiar with the typical characteristics of a given species, you will be able to decide whether or not the notable characteristics for a cultivar are important to you, or not. Sometimes "any old" seedling of the plain species suits the gardener's purpose just fine, in other cases a cultivar is in fact of benefit. It really depends on why (and where) you are putting the plant as well as personal preference.
I hope this answers your question. Since you are considering using natives in your yard, you might want to concentrate on those that are specifically suited to your microclimate and soil as well as being locally adapted, meaning with a provenance close to home.
When you select the plant, you will want to use the same guidelines as ever meaning the growing conditions need to be suitable to that particular plant's needs in terms of things like available space, amount of sunlight, wind exposure, soil type and moisture, and so on.
Many natives are native to a very broad geographic region, eg "the eastern United States" and as such localized populations may have developed certain tolerances for certain climate and/or soil conditions. For this reason, for example, a redbud tree from a northern source would probably have better cold tolerance but lesser heat tolerance than a redbud tree grown from seeds collected in the warmer southern areas. Hence locally collected seed or locally selected plants would be more likely to do the best in your locale.
Also, in my opinion, it is generally a good idea to purchase only nursery-propagated plants rather than those collected in the wild at the expense of a native population. This can be of significance especially in the wildflower trade.
Good luck with your project, I'm sure you will enjoy your native plants.
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