The Entity Id

The entity ID is a value which uniquely identifies individual grids and blocks. You can access it through the item's EntityId property. This value is guaranteed to remain the same during the entire lifetime of the item in question, even if you reload your world.

Know when an entity is recreated

There are caveats. Blocks are pretty clear cut, as long as they exist they will have the same ID. Grids are a bit harder to deal with though.

The Merge Block is very popular to use as a docking clamp, and people are often not aware of the fact that this isn't really what this block was designed to do. It's original purpose was to permanently merge grids together - hence its name. When you lock your grids together with a merge block, all the blocks in one of the grids are moved to the other grid, and then the first grid is destroyed. When you unlock again, the game creates a new grid, and moves blocks from the merged grid over to this new grid. This means that the entity ID of the grid your programmable block is placed on might change whenever you use a merge block. This is something you need to be aware of as a scripter if you intend to make use of this value.

Blueprints and Copy Paste

This one should be rather obvious. When you paste in new grids from either blueprints or a copy/paste in creative, all grids and blocks will get new IDs compared to the original copy or blueprint.