Hello Iraklis
The is-type command is used to configure the routing level for an instance of the IS-IS routing process. The command is applied under the ISIS routing config mode like so:
R1(config-router)#is-type level-1
The circuit-type command is used to specify the type of adjacency that will be created and is configured using this syntax in interface configuration mode:
R1(config)#interface GigabitEthernet 0/1
R1(config-if)#isis circuit-type level-2-only
The circuit-type command essentially allows you to configure specific interfaces to send out and accept only particular types of hellos that will create the desired adjacencies. For example, you can configure a router to have some interfaces to be level 2-only to prevent wasting bandwidth by sending out unused level 1 hello packets. The circuit-type command should only be used on ISIS routers that are between areas, that is Level1-2 routers.
The is-type and circuit-type commands are related, but they do two different things.
For more info on these commands, take a look at these Cisco command references:
https://www.cisco.com/c/en/us/td/docs/ios/iproute_isis/command/reference/irs_book/irs_is1.html#wp1013280
https://www.cisco.com/c/en/us/td/docs/ios/iproute_isis/command/reference/irs_book/irs_is1.html#wp1014983
I hope this has been helpful!
Laz