Here is a quick solution:
%% Set up the Import Options and import the data
opts = delimitedTextImportOptions("NumVariables", 7);
% Specify range and delimiter
opts.DataLines = [3, Inf];
opts.Delimiter = " ";
% Specify column names and types
opts.VariableNames = ["VarName1", "VarName2", "VarName3", "VarName4", "VarName5", "VarName6", "VarName7"];
opts.VariableTypes = ["categorical", "double", "double", "double", "double", "double", "double"];
% Specify file level properties
opts.ExtraColumnsRule = "ignore";
opts.EmptyLineRule = "read";
opts.ConsecutiveDelimitersRule = "join";
opts.LeadingDelimitersRule = "ignore";
% Specify variable properties
opts = setvaropts(opts, "VarName1", "EmptyFieldRule", "auto");
% Import the data
lageos1 = readtable("lageos1.txt", opts);
% Select and separate data
D=lageos1;
DS = D((lageos1(:,1).VarName1=='PL51'),:); % Selected data
