Being a Router requires a lot of memory and current because it needs to keep track of other nodes in the network and has its radio on continuously. End Devices are typically memory and current constrained. Hence, dedicated End Devices cannot become routers. However, a Router can act as an End Device.
From the documentation in my link:
Router
A Router is a node that:
forwards packets for network devices
provides secure commissioning services for devices trying to join the network
keeps its transceiver enabled at all times
End Device
An End Device (ED) is a node that:
communicates primarily with a single Router
does not forward packets for other network devices
can disable its transceiver to reduce power
Device types
Furthermore, nodes comprise a number of types.
Full Thread Device
A Full Thread Device (FTD) always has its radio on, subscribes to the all-routers multicast address, and maintains IPv6 address mappings. There are three types of FTDs:
Router
Router Eligible End Device (REED) — can be promoted to a Router
Full End Device (FED) — cannot be promoted to a Router
An FTD can operate as a Router (Parent) or an End Device (Child).
Minimal Thread Device
A Minimal Thread Device does not subscribe to multicast traffic and forwards all messages to its Parent. There are two types of MTDs:
Minimal End Device (MED) — transceiver always on, does not need to poll for messages from its parent
Sleepy End Device (SED) — normally disabled, wakes on occasion to poll for messages from its parent
