https://developer.arm.com/documentation/100068/0623/Migrating-from-armasm-to-the-armclang-Integrated-Assembler/Data-definition-directives

I think I found the definition of this ARM directive here :

.inst Allocate a block of memory in the code, and specify the opcode. In A32 code, this is a four-byte block. In T32 code, this can be a two-byte or four-byte block. .inst.n allocates a two-byte block and .inst.w allocates a four-byte block.