Transport layer and transport package

Question

I have a fundamental query on transport package and layer.
Objects are classified in packages depending on the project and put in $TMP or test package else they are local ? Only objects belonging to a transport package can be transported , others are not ? How transport package is defined ? All objects must be a part of transport package else can not be transported ?
ABout transport layer, I understand this is used to define which path objects will take during transport particularly when there are multiple transport routes. But what are the advantages of it ? How we can define it (trx code/menu path) ?
Is it mandatory to use transport package and layers ?

Answer 1

Hi Mike,
Just to clarify a few basic concepts:
A transport package (formerly called a "development class", a term you may still encounter) groups related development objects. Whenever you create a new development object (table, program, ...) you have to assign it to a package; the object is then placed into a transport request. Alternatively you can indicate it is a local object (its package is then "$TMP") but such an object is non-transportable.
A transport layer identifies the route that transportable devmt objects must follow. Typically in your system you will have two layers: a layer usually called Zxxx (with xxx = SID of your development system) is used for your own developments. Another layer called "SAP" is used for transporting SAP objects, for example changes to standard SAP code implemented via transaction SNOTE and recorded in repairs. Without the "SAP" layer in place, repairs would be non-transportable.
Each transport package belongs to a layer. You assign the layer when you create a new package in transaction SE80. You can change the layer later on, but this is rarely needed and can cause quite a few side-effects. SAP objects always have layer "SAP". The layers themselves are managed in STMS, under "Transport Routes".
Hope this clarifies things,
Rgds, Mark