The service is an act through which a party notifies an enforcement order to the other party through a Judicial Officer.
This enforcement order can be a judgement rendered by default (in the absence of one of the parties) or after trial (in presence of all the parties) but also a coercion issued by a public authority (Belgian State, city, region, Social Security Services, Social Insurance Organisation, …) or through a social insurance fund without intervention by a judge. It can also be a notarial deed.
Whatever title is pronounced or issued, it shall be served before initiating the enforcement procedure, such as e.g. the seizure of furniture.
The service gives entitlement to the debtor to initiate proceedings (opposition or appeal) in principle within one month following the service, but this time period is sometimes shorter (in criminal matters or in the matter of social security contributions for example). Anyhow, if you want to initiate proceedings, your best option is to consult a lawyer (possibly via the Legal Aid Bureau).
You should note that in some cases (coercion, judgement granting provisional enforcement, …), in order to avoid having to support the charge of both acts, the service may include a payment order. In such case, in principle, enforcement (for example seizure – eviction) could be pursued in spite of your opposition or appeal.
At the deadline of the appeal period and to the extent you don’t react in any way (appeal – payment – application for a repayment plan), the decision will become definitive and the procedure will be continued until the sale of your furniture or of your real estate and/or an attachment of your salary (*).
(*) To the extent, of course, it concerns a recovery file. Instruments can be served in other cases, such as a divorce, custody of the children, accident at work, … In these cases, strictly speaking, there is no enforcement procedure.