TL signifies Truck Load, or more specifically full truck loads in freight terms.

A standard 48 foot trailer will typically have space to haul 24 standards sized pallets of freight, while a 53 footer will hold 26 pallets. If the commodity being shipped is light enough and sturdy enough, pallets can be double stacked to maximize trailer capacity. 

Weight restrictions usually limit the maximum weight load for a standard size trailer up to roughly 44,000 lbs. Some regions have an additional weight allowance of 20,000 or so per extra axle, so a tri axle trailer can haul 60,000 lbs, and quad axle 80,000 lbs. Conversely, smaller straight body trucks will have limits of 16,000 to 24,000 lbs, depending on the model.

At times, regions under thaw rules in spring will have reduced weight limits. Furthermore, the weight must be evenly distributed over the length of the trailer. 

Open weight stations along major highways enforce these limits with strict rules, which include steep fines, forcing unloading or redistributing of weight on the spot, etc.

In general, a full load will be delivered to one destination. Occasionally, the truck will be carrying multiple deliveries,  also referred to as a multi drop TL load.