can you please explain the the concept of Secondary ledger - i don't see any video on that
under what scenarios do we use

If you need a separate representation of your ledger in a different calendar, chart of account, calendar, and/or accounting convention then you can create one or more secondary ledgers and attach them to your primary ledger.
If you ened a separate representation of your ledger in a different currency only then you can create a reporting ledger and attach it to your primary ledger.
System copies accounting activity happening in your primary ledger over to your secondary ledgers.
You must maintain periods for your secondary ledgers just like you maintain periods for your primary ledger. Therefore they come with maintenance cost.
Secondary ledger could have their own reporting ledgers as well.
Secondary ledgers could be created at different levels: Balances Level, Journal Level, Subledger Level, Adjustment Only.
In Balances Level Secondary Ledger only balances (net activity) are copied over using Consolidation Data Transfer program.
In Journal Level Secondary Ledger journals are copied over by Journal Posting programs.
In Subledger Level Secondary Ledger journals are copied over by SLA Create Accounting and Journal Posting programs.
In Adjusmtent Only Secondary Ledger you only enter adjustments in secondary ledger. Complete picture is obtained by combining the secondary and primary ledgers's data in reports. In this case, the primary and secondary ledgers must share the currency, caledar, and chart of account.

Example 1:
ABC is a Canadian company with following Cs:
COA Structure: COMPANY-ACCOUNT-DEPARTMENT-PROJECT
CURRENCY: CAD
cALENDAR: JAN-DEC
CONVENTION: ACCRUAL

This company is acquired by XYZ, which is a US based company.
XYZ requires ABC to maintian its net activity per month in US dollars.

ABC will create another representation of its ledger in US currency. Hence a balance level reporting ledger will be created.

Example 2:
ABC is a Canadian company with following Cs:
COA Structure: COMPANY-ACCOUNT-DEPARTMENT-PROJECT
CURRENCY: CAD
cALENDAR: JAN-DEC
CONVENTION: ACCRUAL

This company is acquired by XYZ, which is a US based company.
XYZ requires ABC to publish its data in US dollar.
Not only that but XYZ also wants ABC to maintain its journal level data in XYZ's chart of account: COMPANY-ACCOUNT-COST CENTER.
Segment values also different between XYZ and ABC for COMPANY and ACCOUNT segments.

ABX will create another representation of its ledger in US currency and XYZ's chart of account. Hence a secondary ledger will be created at either journal level or subledger level.
A consolidation mapping needs to be defined between the two COA structures which will be used by the system to copy the data over.