From the research I have done on SPDs, it appears that it is recommended to layer or cascade the different types (types 1 through 4). For example, here is a video from Schneider Electric which explains the different types of SPDs. This seems to suggest that it is best to use all four types to protect against surges.
If I understand correctly, type 1 do not require an overcurrent protective device (breaker) whereas type 2 do require a breaker. In fact, the requirement of a breaker appears to be the only difference between type 1 and type 2 SPDs. This allows you to install a type 1 SPD on the line side of the home's load center, whereas type 2 SPDs need to be installed on a breaker inside the load center.
From what I understand, SPDs installed on the line side of the load center (type 1) protect from surges on the incoming power, whereas SPDs installed on the load side (type 2) protect from surges generated from inside the home (e.g., from an AC compressor).
But why wouldn't an SPD installed on the line side also protect from internal surges, and why wouldn't an SPD installed on the load side also protect from external surges?
Do type 1 and type 2 SPDs provide different kinds of protection? So if I install a type 1 on the line side of the home's load center (e.g., inside an outdoor rated meter / disconnect combo panel), is any additional protection offered by installing a second SPD (type 2) inside the home's load center on a 20A breaker (which is located on the other side of the exterior wall, just feet away from the meter)?
Specifically, I already own a Square D HEPD80 (type 1) and also a Siemens FS140 (type 2). If I install the type 1 at the meter / disconnect panel (i.e., to protect from incoming power surges), does installing the type 2 inside the main load center add a different type of protection (i.e., internal surges from appliances)? It seems to me that cascading SPDs in this way just adds more of the same protection from both sources of surges.