There are various templates available, summarised here for convenience along with notes on when they should be used (these notes have no official standing).
Full documentation of the templates, and other doding issues, is at https://community.wmo.int/activity-areas/wis/wis-manuals
(309050 is BUFR PILOT format - pressure coordinate, if there are both pressure and height measurements I would suggest using 309052 or 309057)
309051 is BUFR PILOT format - height coordinate, use for wind-only ascents without a pressure sensor
309052 is BUFR TEMP ascent format (original template, 309057 is slightly better now)
309053 is BUFR dropsonde format
(509055 is BUFR TEMP ascent format with height coordinate but not pressure, not used in practice)
309056 is BUFR TEMP descent format
309057 is latest BUFR TEMP ascent format (compared to 309052 there is higher precision for height and pressure, plus extra metadata)
These are sometimes written with spaces, eg. 3 09 057.
It is important that there should be one report containing all data from the ascent, although there can be earlier partial reports (particularly up to 100 hPa) for timeliness reasons.
Ideally there should be an overlap of 2-6 months between good quality BUFR made available over the GTS and the withdrawal of TAC reports - with notification of these changes via https://community.wmo.int/news/operational-newsletter .
Following approval of the plans for the Global Basic Observing Network (GBON) in October 2021, radiosonde data should be reported at high vertical resolution from January 2023 and details of the horizontal/time sampling required. See https://public.wmo.int/en/events/constituent-bodies/cg-ext2021 and https://meetings.wmo.int/Cg-Ext-2021/SitePages/Session%20Information.aspx .