I wouldn't say its nonsense, they are using it to accommodate for the fact that if you had a DLM reset it may take a while to find your lines stable settings such as removing interleaving.
It might not strictly be 10 days, could happen in 48 hours, but its easier for them to stick to the 10 day for all DSL connections than to confuse none technically minded customers and get callbacks from people because their line took longer than usual to stabalise.
There's also the fact DLM gets fiddled with. Recently for example it seems to be far more aggressive about putting interleaving on ECI lines and not wanting to take it off again.
It is nonsense.
The 10 day training period only every existed on the ADSL DLM.
When FTTC launched all lines defaulted to fastpath immediately, with DLM putting the line on the fastest, wide open profile from day 1.
It was only later when G.INP was rolled out to Huawei cabinets that it could then take around 48 hours for G.INP to be applied.
It could actually take weeks or months in some cases for G.INP to be applied.
It's only in the last couple years (many years after FTTC launched) that the DLM can take a little longer to optimise a line with lower SNRM (dB) profiles on Huawei cabinets.
It's even more recent that ECI lines default to Interleaving and can take 48 hours to go to fastpath.
The 10 day training period has been quoted by ISP's for FTTC since launch.
There has never been a 10 day training period on FTTC.
That message wasn't kept to stop confusing customers because the DLM had optimisations to do anyway so it's easier just to keep it there.
DLM making optimisations to FTTC lines after activation only happened years later.
FTTC lines always started at the full potential sync speed with DLM acting on a line immediately.
It was just a convenient excuse to fob off customers for the 1st 10 days.
With MaxDSL the process was very different.
Maximum Stable Rate. (MSR) - Training / Stabilisation Period.
During the first 10 days on MAX details of your logon/sync events are recorded in an Event Collector which every 15 minutes then sends this information to RAMBO.
RAMBO collates events from all users and analyses each users data over a 10 day period to find the Maximum Stable Rate (MSR) for their particular line. The MSR is set from the lowest achieved rate over this 10 day period rounded down to the nearest 0.5Mb. A notification after this period is sent to the RAP profile and your ISP.
The 10 day training period is often referred to as the "Stabilisation Period", and will only commence once a line has been in sync for 15 mins or more - not necessarily the date from when the line is maxed. There is a rolling 10 day period which allows flexibility if for some reason sync is not attained during the immediate period after the line being maxed (such as holidays and the router is switched off).
There's been a lot of talk/hype about the MSR, but in day to day use it doesn't really do much as its main purpose is to set your Fault Threshold Rate.
Your MSR will generally stay the same throughout MAXdsl, although a BTw engineer can request the MSR process can be restarted. There has also been some rumours that the MSR process can be restarted during migration but this is not the case. particularly in the when of migrating between 2 IPStream ISPs.
There is no such thing on FTTC, at all.
The DLM is in full swing from day 1.