Formula reference with apostrophes questions

Hello! I have a consolidation table with multiple sheets. When making a formula to use values from certain sheets the formula appears with apostrophes' (e.g. ='PL_EUR'!S150), while for some sheets without (i.e. =BS_EUR!R34).

Can you somebody please explain:

1) What do apostrophes means in this case?

2) How do I make all sheets to make formula with apostrophes in the formula?

I questions arose, because when cutting and pasting the main sheet rows and columns (with formulas to different to sheets), the formulas to sheets without apostrophes start appearing as #REF.

='PL_EUR1'!Y11+PL_EUR2!Y12 now is ='PL_EUR1'!Y11+#REF