GL Important Concepts and Questions

1. Which process push the data into which tables?

Create Accounting : Subledger tables and xla_events -> xla_ae_headers, xla_ae_lines and xla_distribution_links
Transfer to GL: xla_ae_headers and xla_ae_lines -> gl_je_headers and gl_je_lines
Post to GL : gl_je_headers and gl_je_lines -> gl_balances
Subledger Accounting Balances Update : xla_ae_headers and xla_ae_lines -> xla_control_balances

2. Which tables store segment values and descriptions?

The tables are:
- FND_FLEX_VALUES_TL
- FND_FLEX_VALUES.

3. Why is the Field still protected against update, even after unfreezing the structure?

Most probably another flexfield that uses this value set is still frozen.
You can find the other flexfields with the following sql which shows the frozen structures containing the value set:

select vs.flex_value_set_name, st.id_flex_structure_code
from fnd_id_flex_structures st, fnd_flex_value_sets vs, fnd_id_flex_segments sg
where st.id_flex_num = sg.id_flex_num
and vs.flex_value_set_id = sg.flex_value_set_id
and st.freeze_flex_definition_flag = 'Y'
and st.application_id = 101
and st.id_flex_code = 'GL#'
order by 1,2