PRELOAD flag seems to cap available historical data to maximum 2 years (current and previous).
Code
void run()
{
	// set(PRELOAD);
	
	Verbose 	= 3;
	BarPeriod 	= 60;
	LookBack 	= 10000;
}
Log:
Quote
testlb
Login 0 Oanda V20..
!account 101-004-9964544-004 at UTC 01-03 12:49
V 2.600 on Wed 24-01-03 15:49:28
Load AssetsFix
Warning 054: EUR/USD LotAmount 1000 -> 1.0
Warning 054: EUR/USD PIPCost 0.1000 -> 0.0001000
!EUR/USD 01-05 20:49:30 to 01-03 12:49:30
!=> 12-14 02:00:00 to 01-03 13:00:00, 300 ticks
Load EUR/USD prices..
!EUR/USD 01-05 20:49:30 to 12-14 01:30:00
!=> 11-27 15:00:00 to 12-14 02:00:00, 300 ticks 14951 h
EUR/USD 14951 ticks read
14581(23717) bars 2021-08-10..2024-01-03 generated
EUR/USD: 0..23716

Trade: testlb EUR/USD 2024-01-03
Loading Data\testlb_d.trd (V2.600)
Lookback 10000 bars, 2022-05-11..2024-01-03

Bar 14580 continued until 13:00:00

End of lookback period at 12:49:54
[Stop] at 12:49
Logout.. ok

So far all good. Now let's set PRELOAD flag.

Available data: EURUSD 2005 - 2014 (including).
Code
void run()
{
	set(PRELOAD);
	
	Verbose 	= 3;
	BarPeriod 	= 60;
	LookBack 	= 10000;
}
Log:
Quote
testlb
Login 0 Oanda V20..
!account 101-004-9964544-004 at UTC 01-03 12:47
V 2.600 on Wed 24-01-03 15:47:03
Load AssetsFix
Warning 054: EUR/USD LotAmount 1000 -> 1.0
Warning 054: EUR/USD PIPCost 0.1000 -> 0.0001000
!EUR/USD 01-02 19:34:00 to 01-03 12:47:04
!=> 12-14 02:00:00 to 01-03 13:00:00, 300 ticks
Load EUR/USD prices.. 374729+18 h
EUR/USD 374747 ticks read
6098(15263) bars 2023-01-02..2024-01-03 generated
Error 047: EUR/USD 2023-01-02 2024-01-03: 6098 bars, 2759 skipped, 10000 needed)
EUR/USD: 0..15262

Trade: testlb EUR/USD 2024-01-03
Loading Data\testlb_d.trd (V2.600)
Lookback 10000 bars, 2023-01-02..(NaD)

Bar 6097 continued until 13:00:00

End of lookback period at 12:47:08
[Stop] at 12:47
Logout.. ok



Interestingly if I delete EURUSD_2024.t6 file, PRELOAD flag is ignored and even 2021 - 2023 data is downloaded from broker.
Code
void run()
{
	set(PRELOAD);
	
	Verbose 	= 3;
	BarPeriod 	= 60;
	LookBack 	= 10000;
}
Log:
Quote
testlb
Login 0 Oanda V20..
!account 101-004-9964544-004 at UTC 01-03 12:57
V 2.600 on Wed 24-01-03 15:57:17
Load AssetsFix
Warning 054: EUR/USD LotAmount 1000 -> 1.0
Warning 054: EUR/USD PIPCost 0.1000 -> 0.0001000
!EUR/USD 01-05 20:57:19 to 01-03 12:57:19
!=> 12-14 02:00:00 to 01-03 13:00:00, 300 ticks
Load EUR/USD prices..
!EUR/USD 01-05 20:57:19 to 12-14 01:30:00
!=> 11-27 15:00:00 to 12-14 02:00:00, 300 ticks 14951 h
EUR/USD 14951 ticks read
14581(29949) bars 2021-08-10..2024-01-03 generated
EUR/USD: 0..29948

Trade: testlb EUR/USD 2024-01-03
Loading Data\testlb_d.trd (V2.600)
Lookback 10000 bars, 2022-05-11..2024-01-03

Bar 14580 continued until 13:00:00

End of lookback period at 12:57:41
[Stop] at 12:57
Logout.. ok