I want to store large number of lines ( thousands ) . So , what is more convenient - initializing a variable and storing in it or creating a label and making if invisible and then storing the data in it.
I would say a variable, (maybe a list). When you are using a label, you are interacting with the UI, and even if it is in the background, because of the volume there could be unforseen consequences. i.e. you may see it flicker when the UI changes etc.
There’s no doubt about it. Storing means saving for later use. A label is for displaying information. Unless you are planning on displaying all that information on the screen at once, a label is not the right way to store data.
Like @Cian_O_Sullivan said, a list would be the optimal solution, being maybe every line an item of that list. And that will also allow you to access each line independently in an easier way.