I will try to explain. The crafting in LE is very deterministic. It works as follows:
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Every item can have up to 4 affixes. (2 primary (usually offensive) and 2 secondary (usually defensive)). These affixes can have up to 5 tiers (levels basically). E.g. Fire Damage affix might give +10% Fire damage per tier.
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If you disenchant any item (which you can do on the move) you have a chance to get shards corresponding to one or more of the affixes that item had (higher affix tiers = more shards). Some affixes are common, some are very rare etc. This means every item is potentially valuable, even blues (2 affixes) from a disenchant point of view.
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These shards go straight to your bank and are shared by all your toons. Any shard can then be applied to any other item, and will add 1 tier of that affix to the new item (or increase the tier if the new item had that affix already).
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The only non-deterministic thing in this is the amount of “forging points” on the item that get used during the craft (this is the RNG part but it can be mitigated with a glyph (a glyph is a crafting modifier item; there are tons of them that do different things to affect your craft)).
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This all means that the definition of a good base item to craft with is one which has as many affixes of types that you actually want, and as few affixes that you do not want, plus as many forging points as possible.
Even then, this all just scratches the surface. Crafting can be modified in many ways with glyphs including removal of affixes, chance to lock an affix and create a new affix slot (hence you can have more than 4 affixes on items), chance to score a crafting crit (the craft uses zero forge points) and on and on and on and on.
One of the really nice things you can do is convert a 4 affix rare into a Legendary, IF that rare has the same base item type and sufficient “legendary potential” points. This creates a new copy of that Legendary, but it also inherits some of the affixes on the rare that crafted it, making it an uber-Legendary perfect for your build.
Since one of the affixes in the game is going to be “+1 skill point for skill X”, this is basically how you’d get +1 Storm Totem onto a Legendary.
Crafting in LE is a game all of its own and is very very satisfying. Best crafting system I’ve ever encountered, by far.