It depends on what p1 is. If p1 is a struct variable, p2 would be a field of p1. If p1 was a classdef OOP object, then p2 could be a property or a method of p1. E.g., for a struct:
>> p1.p2 = 1:3 % <-- Create a struct called p1 with one field called p2
p1 =
p2: [1 2 3]
>> class(p1) % <-- The class of p1
ans =
struct
>> class(p1.p2) % <-- The class of the field element p2
ans =
double
>> p1
p1 =
p2: [1 2 3]
>> p1.p2
ans =
1 2 3
