Duty cycle generator (PWM Modulator)
Being the author of these free ready-made SIMPLIS templates, I can answer the questions. When the PWM artificial ramp is of 2-V amplitude, you have to limit the maximum voltage excursion of the op-amp to limit the duty ratio. For instance, with a 2-V peak voltage and a maximum duty ratio arbitrarily fixed at 80%, then the maximum excursion applied at the PWM comparator (+) pin must be: 2 x 0.8 = 1.6 V. To respect this upper limit, you have many options. Below are a few I can think of:

directly clamp the op-amp output maximum voltage. But is does not really reflect what a real op-amp will do. In many ICs, the internal op-amp is clamped to the internal \$V_{dd}\$ rail which is often set at 5 V. Besides, clamping a voltage source is not easy unless you resort to a Zener diode which I don't like since you need a resistance and precision can be at stake.
internally clamp the voltage before it reaches the (+) pin. This what I've done here. The 1-S voltage-controlled current source associated with the 1-\$\Omega\$ resistance provides a gain of 1. I could have used a 100-µS instead with a 10-k\$\Omega\$ resistance and the same unity gain would have been provided. Now, to clamp the voltage excursion of the internal circuit, I can directly install across the output a 1.6-V Zener diode - which is not common - so I need to create my own.

SIMPLIS offers the ability to tailor non-linear piecewise linear (PWL) elements like resistances for instance. That way, you can describe the exact response you want based on voltage and current values. In my case, I have defined a perfect diode (almost zero dynamic resistance, no capacitance etc.) with a 1.8-V threshold. The diode does not conduct (2 pA) until the voltage across it terminals reaches 1.8 V. At this moment, the current is almost infinite and it is a sharp clamp. A simulator like SPICE could be bothered by this discontinuity but SIMPLIS isn't and it works very well. The (+) symbol is the anode of the diode:

You realize that connecting this diode directly across the zero-ohm output impedance of the op-amp would generate a lot of current when clamping while you set the maximum current via the gain in siemens of the voltage-controlled current source. Having access to these PWL diodes is very practical and lets you build all kinds of limiters for instance. The insertion of the clamping circuit independently from the op-amp lets you select any type you want without caring about the maximum duty ratio limit that you set in the downstream circuitry.