Check out the following link from Microsoft: http://azure.microsoft.com/en-us/documentation/articles/service-bus-dotnet-how-to-use-topics-subscriptions/?rnd=1
Ctrl+f "Create Subscriptions with Filters" and read that section.
In short: Subscriptions can be created with SQL-like filters that run on message properties (standard or user-specified). The subscription will only pull messages that pass its filter. However, if a subscription doesn't have a filter, it will receive all messages. If you control the management of the messaging entities, then you can put a filter on every subscription so they only receive the messages meant for them, otherwise the data might be received by someone it wasn't meant to be sent to.
Depending on the data sensitivity, you may also want to encrypt it before publishing it to the topic.