Using the term "heavy" gives a false sense of what line infantry was like in the period. In Roman and Medieval times it was typically used to denote units that had stronger armor and better weapons. However, with the proliferation of gunpowder weapons and nations ditching armor for most of their troops, this term also fell out of favor for the most part. You will still see it show up with cavalry units such as the French Cuirassiers, with their name referencing the metal Cuirass they wore on their chest. Since foot infantry in the period didnt wear armor, it doesn't really apply to them though.

The more appropriate term is just Line infantry (with each nation of its own variation in their language). Terms like "Shock troops" or "Elite" units are usually more appropriate for distinguishing different units for different task other than just normal Line infantry. Skirmishers are usually termed light infantry for two reasons. One being they are less numerous in an army than normal line infantry. The second is the formations used by skirmishers. Normal line infantry fight in lines or column formations. Skirmishers fight in loose formations, using cover such as hills, trees, and rocks to fire at the enemy behind. Its a lot closer to modern warfare compared to what line infantry was used for in the period. They used their concealed positions to shot at enemy formations to cause panic, slow them down slightly, and in some cases target officers to cause confusion.

"Shock" troops of the period are often called heavy units in the modern day out of ease. They didn't wear armor and they didn't necessarily fight in "heavier" formations. Most of them were simply prestigious units for reasons such as unit history, connections to royalty such as royal guard units, or units consisting of war veterans. Their "shock" value came more from their presence and the enemies perception of them which is used to "shock" the enemy and break their morale. The most famous of these is the French Old Guard which was part of Napoleon's Imperial Guard, nicknamed "The Immortals" by many French soldiers. Napoleon was reluctant to use them in battle, usually saving them until the very end to secure victory and sweep the enemy from the field. The Old Guard had better uniforms, food, and requirements to join to make them special and stand out from other Line Infantry. The be part of the Old Guard one had to be above average height at at least 5 feet 10 inches, at least 10 years of service, be under 35, served in at least 3 campaigns, and have fought the enemy at the front. This meant that they were some of the most experienced soldiers as well as the tallest on the battlefield.