I think everyone has his own methods.
For me, globals are last resort.
Instance variables are very usefull within layout. To identify instances and set/get instance status.
If I have more instances frome same type to set to something, I use locals. Because you cant pick from same type twice.
Finally, to have a database over all layouts, there are dictionary and array, but for me they are to inflexible, so I build my own.
https://www.scirra.com/forum/plugin-behavior-directdata_t147493