Updated with Source variable, for tutorial part 2.

StatModifier.cs

public readonly float Value;
public readonly StatModType Type;
public readonly int Order;
public readonly object Source; // Added this variable

// "Main" constructor. Requires all variables.
public StatModifier(float value, StatModType type, int order, object source) // Added "source" input parameter
{
    Value = value;
    Type = type;
    Order = order;
    Source = source; // Assign Source to our new input parameter
}

// Requires Value and Type. Calls the "Main" constructor and sets Order and Source to their default values: (int)type and null, respectively.
public StatModifier(float value, StatModType type) : this(value, type, (int)type, null) { }

// Requires Value, Type and Order. Sets Source to its default value: null
public StatModifier(float value, StatModType type, int order) : this(value, type, order, null) { }

// Requires Value, Type and Source. Sets Order to its default value: (int)Type
public StatModifier(float value, StatModType type, object source) : this(value, type, (int)type, source) { }