This does not mean anything special, a suspended transaction is just a transaction that is temporarily not used for inserts, updates, commit or rollback, since a new transaction must be created due to the specified distribution property, and only one transaction can be active at a time.
Basically, there are two transaction models: a nested model and a flat one . In a nested model, if you start a transaction and you need another, the first remains active, that is, the second will be nested inside its parent and so on. On the other hand, in a flat model, the first transaction will be suspended, that is, we will not use it until a new one is completed.
AFAIK the flat model is used almost exclusively (including Spring and the EJB specification), since it is much easier to implement : at any given time there is only one active transaction, so it’s easy to decide what to do in case of a rollback, say, due to an exception. More importantly, the underlying database should support it if you need a nested model, so the flat model is then the common denominator .