I fell unsure about understanding the Scope of workbooks and worksheets

Context of question:
Within one module associated with a single workbook
Within a single subroutine
If I open an existing workbook and existing worksheet within the open workbook
assigning a variable to each of the above objects

then make a function call within the above single subroutine
I am assuming the code inside the function call
Has no idea which workbook or worksheet is open

Hence I assume I must pass the variable name for the workbook and
worksheet as parameters to the function call and use them to extract
data from the worksheet

Question:
Do I understand the scope of workbooks and worksheets within a single subroutine?