Milestones

Milestones are goals that rewards the player items or gifts, when they achieve the goal. 

Pet Milestones
Pet Milestones are milestones that reward the player Pets when they achieve them.

Dolphin Pet :

It is obtained by killing Sea Creatures.

The milestones are as follows;

Rock Pet :

it is obtained by mining Ores. Stone, Cobblestone, Endstone does not count.

The milestones are as follows;

Dungeon Milestones
Dungeon Milestones are class goals that give the player the possibility to gain better loot and to earn more class XP after clearing a Dungeon.

The Milestones vary for each class and are greater on higher floors;

Achieving Milestone 2 is required to open the Post-Boss Dungeon Chest,you'll also get reduced Catacombs and Class Experience if you fail to reach Milestone 3 before the dungeon run ends.

Bestiary Milestone
The Bestiary Milestone allows players to obtain rewards based on the amount of kills they've accumulated on Mobs in different areas. The progress on this milestone do not increase with the same Mob types in those different areas. For instance, an Enderman on your Private Island will not count towards your total kills on Endermen in The End.

Do note that rewards obtained from the 81 individual Family milestones. These family milestones offer you rewards such as strength, an increase in coins, magic find and experience orbs gained from killing that Mob, as well as information on what items they drop. (This table is not complete. Please add any information that is missing.)

Other Milestones
The Enchantment uses the milestone system to upgrade.

It is upgraded by killing Sea Creatures while having the Expertise enchantment on your rod.

The milestones are as follows;

Trivia

 * While players cannot see their Milestones progress, their progress can be retrieved from SkyBlock API.
 * Fishing rods that have been enchanted with store expertise_kills in their NBT data.
 * You can easily look your progress up on sky.shiiyu.moe, which is a stats tracker.