Hi,
You are required to buy either a JSB, JSE or JSB-L license when buying a SRX300 series firewall.
JSB: Firewall, routing, NAT, VPN and Junos "basics" (automation etc.) and MPLS
JSE: JSB + Application Security as a perpetual license
JSB-L: JSB but limited to 200 Mbps throughput and 40 Mbps IPsec/NGFW
On top you can then buy subscription licenses for NGFW features (and these will include application security so not much need for JSE).
For now the JSB/JSE/JSB-L features are not enforced but should be during 2017. Subscription licenses are enforced.
MPLS was originally only in JSE but was moved to JSB some time in Q3 2016.
I hope this answers your question.