Raab (Rabb means,the Creator,the Sustainer,the Provider, & the one in whose
hands is the disposal of all affairs)..

Linguistically the word rabb means master, owner or one who sets about
correcting and purifying. When used in a possessive or conjunctive (idaafah)
structure it can be applied to other than Allaah, for example it is said
rabb ad-daar or the master of the house, similarly in the Qur`aan it is
mentioned that Yusuf (AS) said to one of the inmates of prison,
"Mention me in the presence of your master." [Yusuf (JOSEPH) (12): 42]
Likewise it is mentioned in the famous hadeeth of Jibreel, when the Prophet
(SAW) listed some of the signs of the Hour,
.When the slave-girl gives birth to her master (rabbatahaa) [Saheeh Muslim
[Eng. Trans. 1/1 no. 1]]

However the word ar-Rabb can only ever be applied to Allaah. It is one of
the Names of Allaah and means the One Who nurtures and sustains all of His
servants through regulating the affairs and granting all types of favours
and blessings. More specifically He is the One Who nurtures and sustains his
sincere friends by correcting and purifying their hearts, souls and manners.
This is why their supplications are frequently made with this Noble Name
because they seek this specific nurturing. [As-Sa`dee, 'Tayseer al-Kareem
ar-Rahmaan' [pg. 16 ]]

As-Sa`dee said,
The Lord is the One Who nourishes and sustains the whole of the Creation,
meaning everything aside from Allaah, by the very fact of His creating them,
and His preparing for them all that they need and His favouring them with
great blessings which if removed would also remove any possibility of the
creation surviving. Therefore every blessing they possess then it is from
Him, the Exalted.
His sustaining His creation is of two types: General and Specific. As for
the general then it is His creating the Creation and granting them
provisions, and guiding them to that which would benefit them in order to
have them survive in this world. As for the specific then it is His
sustaining His friends with faith and making them conform to it, perfecting
and completing it for them, repressing all that would make them turn away
from it, bridling any hindering factors that may be set up between them and
Him, and safe-guarding them from all evil. It is possible that the reason
behind the fact that most of the supplications made by the Prophets employed
the word 'Lord' was due to it carrying this meaning, for indeed all of the
things they desired through their supplications fell under His Specific
Lordship. [Ibid. [pg. 22]]


Reference:
Tafseer of Surah al-Fatihah, compiled by Abu Rumaysah,(This commentary is a
compilation gathered from a number of commentaries written.) These being:
Tafseer at-Tabaree
Tafseer al-Qurtubi [Jaami lil Ahkaam al-Qur`aan]
Tafseer ibn Katheer
Tafseer ash-Shawkanee [Fath al-Qadeer]
Tafseer as-Sa`dee [Tayseer al-Kareem ar-Rahmaan]
Tafseer ash-Shanqeetee [Adwaa al-Bayaan fee Eedaah al-Qur`aan bil Qur`aan].