BT's SIN 498 document has this to say on the matter:-
2.2.1 Dynamic Line Management
Dynamic Line Management (DLM) is employed in GEA-FTTC. DLM constantly manages lines to maintain a target link quality (speed and stability). It does this for as long as the product exists.
At provision, the line is put on “wide open” VDSL2 line profiles allowing the upstream and downstream line speeds to run at the upper limit of the product option selected.
On the first day of operation, DLM will intervene if severe instability is detected. Otherwise, DLM will wait until the day after provision before deciding if it must intervene, provided that the line has been trained up for at least 15 minutes during the preceding day.
If DLM intervenes it will set a profile with a maximum rate and a minimum rate, where the minimum rate is set at approximately half of the maximum rate. The purpose of the minimum rate is to ensure that the line does not train at a rate which is significantly below the level the line should be able to achieve. If this happened, then the line is likely to remain at a very low rate until a re-train is forced by the user by powering off the modem.
Note : It is the DLM system that sets the line profile, and this should not be interfered with by CPs/users setting rates, SNR margins etc. at the modem.