Talent grades:

Purple - higher chance of having one or two extra move slots and reaching the peak

Blue - possible to have an extra space in the move set limit + higher potential

Green - Standard move set limits + potential to grow beyond level 50, greater growth speed than yellow

Yellow - Standard move set limits + potential to grow beyond level 50

Orange - less move set slots than normal and limited talent unless greater resources are used

Red - worst talent and low move set slots

Those with higher grades of talent should have some minor differences in stat growth during levelling but not by a massive margin that it makes that big of a difference in later levels unless additional items have been used to further raise them.

The higher grades of talent graded Pokémon have a much higher chance of learning egg moves.

Some talents can lower due to past injuries, poor raising of them, mistreatment or bad environmental factors but can be recovered through long term treatment.

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Items & Grades:

Low - they do the job but are less effective and have no additional bonuses

Medium/Normal - using some items could have some effects on improving some stats or even talents depending on the item used

High - much less chance of failure and being ineffective while providing a guaranteed boost in other areas if they are compatible with that Pokémon. E.g. Evolution Stone

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Levels:

1-19 Lowest/Grunt Rank

20-39 Medium/Officer Rank

40-59 High/Elite Rank

60-79 Pseudo Legendary/Champion Rank

80-99 Legendary/Master Rank