Debate no more Hammer…
C1 stands for Compound 1, and it’s the hardest tyre in the 2019 Pirelli range, sitting just below the 2018 hard in terms of compounding. It’s designed for circuits that put the highest energy loadings through the tyres, which will typically feat.
C2 means Compound 2, effectively last year’s medium tyre. A versatile compound, but sitting at the harder part of the spectrum, it comes into its own on circuits that tend towards high speeds, temperatures, and energy loadings. This tyre has demonstrated an ample working range and adaptability to a wide variety of different circuits.
C3: This tyre is equivalent to the soft that was nominated in all but four of the races last year. It strikes a very good balance between performance and durability, with the accent on performance. It’s a very adaptable tyre that can be used as the softest compound at a high-severity track as well as the hardest compound at a low-severity track or street circuit.
C4: This is closest to the 2018 ultrasoft and it works well on tight and twisty circuits. It has a rapid warm-up and huge peak performance, but the other side of this is its relatively limited overall life.
C5: The softest 2019 compound is the heir to the universally-popular hypersoft: the fastest compound that Pirelli has ever made. This tyre is suitable for all circuits that demand high levels of mechanical grip, but the trade-off for this extra speed and adhesion is a considerably shorter lifespan than the other tyres in the range. Getting the most out of it will be a key to race strategy.
Nicely updated site and info from here: F1 tyres: details and technical data | Pirelli
That said, I agree with you that the C1 will see very little use, even if it isn’t quite as hard as last year’s Hard.
(And no, I don’t know what the C1 “will typically feat.” - that’s how it’s displayed on the site.)