Bigquery. What does “Adding NULLABLE or REPEATED columns at the end” mean?

Question

I want to update table in Bigquery. Here is a documentation that says what can and can not be done: https://cloud.google.com/bigquery/docs/tables#updateschema It says:

Allowed operations include:

Adding NULLABLE or REPEATED columns at the end
Making REQUIRED fields NULLABLE

I was able to update table by inserting NULLABLE and REPEATED fields, nested in my schema.

I don't understand what does "at the end" part mean. Isn't it an internal detail about how Bigquery stores data?

Answer 1

I guess documentation talks about logical order that you see when browsing table or doing "select *". The physical storage is implementation detail, but logical order is visible.

Answer 2

You can add columns only to the end of the structure only. You cannot reorder columns or add a column in the middle, like 2nd column.