Explanation of formulas containing "TL" reference

Have a workbook that was sent to me, linked to another workbook. Have just clicked into a cell to see the reference (there is no value in the cell, it is just coloured) to find this: ='[wwtrial M5 J14-20.xls]TL''s'!D5 I've not seen the TL''s'!D5 part before - can anyone tell me what it means??

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='[wwtrial M5 J14-20.xls]TL''s'!D5

[wwtrial M5 J14-20.xls is the workbook name
TL"s is the worksheet name
D5 is the cell being referenced in that worksheet in that workbook.