Is sexual availability, being available to your partner any time he or she wants, part of your submission? Why or why not? Are there limits to this?

Most definitely.

So much so it is part of rule #4.
“I will completely surrender my heart, mind and body to Daddy. My body belongs to him and he can demand access at any time it is appropriate to do so.”

Part of my submission to him is to give him ownership of me. This means that he can have access to me at any point and he exercises this right quite often. I could be standing making a cup of coffee and he will come up behind me, kick my legs open and slid his fingers inside me. He has the ability to tell me to get on my knees at any point (within reason) and I will happily suck his cock if instructed or present myself to him so he can do whatever he likes to his property.

I say within reason as there are of course limits to this which he has never pushed. When I am with my son for example, Daddy wouldn’t ever try to touch me intimately when he is with us and he knows I would never allow him to touch me like that in front of others i.e. family/friends or public. That would take it out of the realm of fun for me and he wouldn’t violate my boundaries like that.