This code defines a TypeScript constant variable `projectType` with a...

This code defines a TypeScript constant variable projectType with a type annotation that restricts its possible values to one of the two string literals: 'Transmission' or 'Distribution'. Here's what it does:

1. The variable projectType is declared using let, meaning its value can change (mutable).
2. Its type is explicitly set to a union type, 'Transmission' | 'Distribution'. This means projectType can only hold one of these two specific string values.
3. The variable is initialized with the value 'Transmission'.

Summary:

This code declares projectType as a mutable variable with strict type restrictions, initially set to 'Transmission'. If someone tries to assign a value other than 'Transmission' or 'Distribution' to projectType, TypeScript will throw a type error.