Filtering, and the sediment that is not dirt
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What it is
Minerals that were dissolved in the sap and are no longer soluble once the sugar concentration climbs. They settle as a gritty deposit and they cloud the syrup. A buyer reads both as contamination, and a graded product is required to be free of cloudiness and sediment.
Filter hot
Syrup is far less viscous hot, so it passes a filter that a cooled batch will not. Cooling and reheating to filter costs quality and time, so filtering belongs in the same operation as the finish rather than as a separate job the next morning.
It is not a safety step
Worth being clear, because makers sometimes treat the filter as the hygiene control. It is not. The safety of the product comes from the sugar concentration and from packing hot into a clean container. Filtering is about clarity, texture and grade.
Where it goes in the day
Finish, measure, filter, pack, all while hot and in that order. Measuring after filtering rather than before is the better habit, because what you bottle is what the label describes and the filter is between the two.
What a formulated syrup needs instead
Dissolved sugar does not throw this sediment, so a simple or bar syrup usually needs only a strainer for whatever flavoring went in. Fruit syrups are the exception and the decision there is cosmetic: a cloudy fruit syrup is a legitimate product if the label does not promise otherwise.
Budget for the consumable
Filter material is used up, and the temptation to push one further than it should go is strongest at the end of a long day with syrup waiting. Costing it properly per batch, and having spares at the bench rather than in a cupboard, removes the decision at the moment you are least able to make it well.