Hello Zaman
The additive keyword will allow you to add an export map without overwriting the previously configured ones. For example, if you configure the following:
PE1(config)#route-map EXPORT_MAP permit 10
PE1(config-route-map)#set extcommunity rt 1:1
and then at a later time you configure this:
PE1(config)#route-map EXPORT_MAP permit 10
PE1(config-route-map)#set extcommunity rt 3:3
The result is that the 1:1 entry will be overwritten by the 3:3 entry. However, if you use the additive keyword like this instead:
PE1(config)#route-map EXPORT_MAP permit 10
PE1(config-route-map)#set extcommunity rt 3:3 additive
Then 3:3 will not overwrite 1:1 but will be added to it.
I hope this has been helpful!
Laz