Spare spool
A spare spool is usually explained by break-offs, and that is the weakest of the arguments: a break-off costs you metres, not the whole fill. Its real point is different — to have a second line on the same reel and change tackle in half a minute.
A second line, not a second reel
The pairing that earns its keep most often is braid and monofilament. One spool carries the sensitive setup, the other the one that needs stretch. The difference between them is set out in the entry on braided line.
The second typical case is the same kind of line in a different diameter: one for light lures, one for heavy ones or for a snaggy bank.
What to check when buying one separately
The spool has to come from its own series, not merely be «the same size». On front-drag reels part of the drag stack lives inside the spool itself, so a foreign spool will either not seat or will spoil the drag — which has its own entry, the reel drag.
Two movements check it: the spool should seat fully without force, and the drag should turn with the same clicks as on the original.
The shallow spool
A shallow spool is the same spool with a lower lip. It is bought for braid, so as not to waste a hundred metres of cheap line filling the bottom: less depth means less backing, or none at all.
It is a convenience rather than a different class of tackle. On a normal spool the same braid goes on over backing and fishes just the same — the filling procedure does not depend on depth.
How to store them
Line on a spare spool ages exactly as it does on the working one, and it sits idle for months while doing so. Keep them dark and dry, and do not leave the fill under tension; the same is said about line care.
And label them. After a season a spool of thin braid and one of thick look identical, and the mistake costs a lost lure.
The lip is what wears out unnoticed
Line leaves the spool over the front lip and rubs on it with every cast. A chip or a burr there cuts line exactly as a cracked guide insert does, and is just as invisible to the eye.
The check is the same — a cotton bud run round the rim: fibres catch where there is a chip. The same trick is described for guides in rod care. A damaged spool is not polished out — it becomes a spare for coarse line, or it is retired.
What we do not state here
How much line a spool of your size holds. The maker prints a capacity for one diameter, and we will not convert it to another: we found no verified formula for that, and an error in the fill is precisely what makes line throw loops later on.