Could someone explain IClaimsTransformation and UserClaimsPrincipalFactory? and which one should be used in different scenario?

I have two code snippets. 

// Sample 1.

public class ApplicationUserClaimsPrincipalFactory : UserClaimsPrincipalFactory<ApplicationUser, IdentityRole>
    {
        public ApplicationUserClaimsPrincipalFactory(UserManager<ApplicationUser> userManager, RoleManager<IdentityRole> roleManager, 
            IOptions<IdentityOptions> optionsAccessor) 
            : base(userManager, roleManager, optionsAccessor)
        {
        }
        protected override async Task<ClaimsIdentity> GenerateClaimsAsync(ApplicationUser user)
        {
            // https://github.com/aspnet/Identity/blob/329eed9e8d14243d0b36385bb1adc9fc85df0e41/src/Core/UserClaimsPrincipalFactory.cs#L82
            var identity = await base.GenerateClaimsAsync(user);
            // Todo... Add or replace identity.Claims.
            return identity;
        }
    }

// Sample 2.

public class ApplicationUserClaimsTransformation : IClaimsTransformation
    {
        private readonly UserManager<ApplicationUser> _userManager;
        public ApplicationUserClaimsTransformation(UserManager<ApplicationUser> userManager)
        {
            _userManager = userManager;
        }

        // Each time HttpContext.AuthenticateAsync() or HttpContext.SignInAsync(...) is called the claims transformer is invoked. So this might be invoked multiple times. 
        public async Task<ClaimsPrincipal> TransformAsync(ClaimsPrincipal principal)
        {
            var identity = principal.Identities.FirstOrDefault(x => x.IsAuthenticated);
            if (identity == null) return principal;

            var user = await _userManager.GetUserAsync(principal);
            if (user == null) return principal;

            // Todo... Add or replace identity.Claims.

            return new ClaimsPrincipal(identity);
        }
    }

I am not clear which sample should be used when manipulate claims at  "// Todo... Add or replace identity.Claims.".  Thanks.