In case you’d rather display quickly the name of an account (instead of creating a collection with only the account in, and looping over it to create it, for instance:
{% assign accounts = period.accounts.400000 %}
{% for acc in accounts %}
{{ acc.name }}
{% endfor %}
you now can do it way shorter:
{{ period.accounts["400000"].name }}
You can combine it even with dots in the account number (for other countries other than BE):
{{ period.accounts["400000.001"].name }}