How should values be selected for the constants for the snubber circuit connected to the triac?

This is a circuit diagram symbol for a variable-capacitance diode.

Variable-capacitance diodes are abbreviated as VCDs. They are also called varicap diodes.
Variable-capacitance diodes has a variable capacitance which is a function of the voltage between anode and cathode.
General-purpose diodes are used for rectification and signal detection, and variable-capacitance diodes are used for RF impedance matching in automatic tuners and transmitters.

snubber circuit

A snubber circuit is inserted for suppressing a suddenly rising voltage (dV/dt) to prevent malfunction. Toshiba recommends RS=47Ω and CS=0.033μF for a 100VAC system, and RS=100Ω and CS=0.1μF for a 200VAC system.
Refer to Toshiba Application Note "Triac couplers-basic properties and application design", section 9.6.