Some doubt of Business Rule

Dear All:

           I am a junior in CSLA framework. There's a doubt when i reading the "Using CSLA 4" about the business rule: It says "the target property can only be used on the ui thread and any use of this property on a background thread will lead to cross-thread exceptions or nasty threading bugs"

My doubt:

1. this rule is only suitable for async rule or both sync/async rule?

2.if i don't have the ui thread, and it just a service to expose interface to others,in this case, can i use the target ?

3. Dose everyone could illustrate some detail example to explain what problem will occur when use target in background thread?

Thanks.