Xanathar's Guide isn't explicit, so let's reverse-engineer it
All of the following conclusions are reverse-engineered from the tables in XGtE and the item descriptions in the DMG. They don't explicitly appear anywhere in the rules.
What is a consumable item?
For the purpose of the following guidelines, a consumable item is an item that can permanently become non-magical or otherwise be destroyed during normal use. An item with charges that can regain them an unlimited amount of times does not count as consumable, even if it can be destroyed by consuming its last charge and rolling a 1 on a d20.
All consumable items are minor items, unless any of the following are true:
- They have an extremely large amount of uses/charges (> 10), typically for a significant effect (gem of brightness, candle of invocation).
- The only way they stop working is being destroyed via HP loss, typically from some direct action from an opposing creature (iron bands of Bilarro, rope of entanglement). Additionally, they aren't expected to always be destroyed the first time they are used.
- They grant a potentially campaign-breaking effect to characters who otherwise would not have it (efreeti bottle).
All common items are minor items.
All non-consumable uncommon items are minor items unless any of the following are true:
- They provide a major combat benefit. For example, they may...
- Grant a bonus to hit, damage, saving throws, or save DCs.
- Summon (or appear to summon) combat-capable creatures.
- Provide in-combat advantage on a common skill, like Stealth or Perception. Removing a disadvantage doesn't count.
- Grant resistance to a type of damage.
- Grant or duplicate combat spells or additional spell slots.
- Grant a flying speed, increase your walking speed, or allow you to walk where you otherwise couldn't.
- Increase one of your ability scores.
- They block high level magic, like scrying.
- They provide a major dungeon crawling benefit, like advantage on thieves' tools checks.
- They provide a major social benefit, like detect thoughts or at-will disguise self.
All non-consumable rare or very rare items are major items, unless they have little or extremely situational combat benefit (folding boat). Cursed items may have a detriment instead of a benefit.
All non-consumable legendary items are major items.
If the item is consumable or is common, rare, very rare, or legendary, it is usually fairly simple to categorize; however, non-consumable uncommon items are a gray area. If they have a major combat benefit, the answer is simple. If not, you end up comparing them to existing items and dreaming up ways they could be used.
Does the wand of secrets provide less of a dungeon crawling benefit than the gloves of thievery? Is it really more important to be able to jump really far (boots of striding and springing) than to see in the dark as a human (goggles of night)? I'm not sure.
What I do know, is that the same people who wrote the XGtE tables decided that attunement-free unlimited flight without an action (broom of flying) is less rare than attunement-required 1 hour flight with an action (wings of flying). You're probably better off using your own discretion for the edge cases.